It seems to me that there are 4 basic questions that any religion needs to be able to address:
(1) Who is God?
(2) If God is all-good and all-powerful, why is there evil in the world?
(3) If God *is* all-good and all-powerful *and* there is evil in the world, what is God doing about it?
(4) If God *is* all-good and all-powerful *and* there is evil in the world *and* God is doing something about it, how do we get with the program?
Catholicism, in a nutshell, is an answer to those 4 questions. What follows is my attempt to give an "in-a-nutshell" summary of the Catholic response.
Who is God?
God is a pure spirit, the Supreme Being, who always has been and always will be; he is omnipotent and omniscient. These and other attributes of God may be discovered by the use of human reason as it reflects philosophically on the universe and the place of man in that universe (see Romans 1:20).
There are attributes of God that can only be known through God's own special revelation, however. These are: (1) While there is only one God, He is a Trinity of Persons; and (2) God is Love. The latter teaching does not mean simply that God is *loving*, but that God is actually Love itself; love defines the very substance of God, such that you could say that God is "made out of" love.
This reality of "God is Love" goes a long way to explaining the Trinity itself. If God really is Love in his fundamental substance, then there are at least 3 things that are co-eternal within God: a Lover, a Beloved, and the Love between them. After all, you can't actually love without someone *TO* love, someone to whom we offer the best of ourselves, who then (ideally) loves us back. If God really *is* love, He lives the same dynamic: the Father is the Lover, the Son is the Beloved, and the Holy Spirit is the Love between them — all co-eternal and sharing the same essence and substance, because all are part of the dynamism of Love that is God.
As for us, we encounter God, even before we are aware of him, as our Creator. After all, if he didn't give us the gift of existence in the first place, we wouldn't be encountering him any other way! The Catholic Church teaches that everything that exists (that is not God) was created by God, and is sustained in existence by God. This latter point is important: the relationship between God and Creation is not like that of a painter to a painting, but more like that of a singer to a song. The painter can hang the painting on the wall and forget about it, but a singer cannot do the same: the moment the singer ceases to sing, the song itself disappears. The song depends on the singer for its continued existence, just as the universe depends on its Creator for its continued existence.
And so why did God create the universe? More importantly, why did he create us? In a nutshell, for the same best reason why a singer sings: for the sheer joy of it. And the amazing thing about the rational creatures that he has created (i.e. angels and human beings) is that they can, in turn, join in the song! The nature of God is intimately joined to the purpose of our existence: by being capable of free will we are also capable of love, which means that we share, in some small way, in the very nature of God. We'll never become other gods, but the fact that God is a communion of loving Persons opens the door to us joining in that communion, growing more and more in godliness for all time — a godliness defined by perfect Love.
If God is all-good and all-powerful, why is there evil in the world?
To tackle this, we need to get a grip on the word "evil". Usually "evil" is divided into two categories: natural evil, and moral evil. The latter arises from an improper exercise of free will, and I think is fairly easy to grasp. The former is more subtle: it, in turn can be sub-divided into physical evil and metaphysical evil. And if at this point you are going "huh?", just stick with me.
Catholic tradition teaches that evil does not actually exist in itself, because evil is actually a privation. In other words, evil is not a "Something", it is a "lack of Something", just like cold is a lack of heat, or blindness is a lack of the ability to see.
In this context, the worst possible evil is actually non-existence itself, i.e. the total lack of any existence at all. The greatest possible good, on the other hand, is to lack nothing, and indeed to not be capable of lacking anything. This is one of the reasons we say that God, the greatest good of all, is necessarily eternal: if He were capable of non-existing, he would not be the greatest possible good. An interesting side-consequence of this is that it also implies that God is, in his nature, a fundamentally joyful being.
Now for us human beings, or indeed any created thing, we are somewhere on this scale of existence. We definitely are not "nothingness", but at the same time we are not "necessary" beings: we *could* fall back into "nothingness" if God were to stop "singing" us into existence. So we possess a true and definite goodness, thanks to the mere fact that we exist, but it *is* a limited existence. The fact of this limited nature in created things is called "metaphysical evil".
"Physical evil" is a bit easier to understand, at least initially. It includes things like meteor strikes, tsunamis, forest fires, disease, and other powerfully destructive forces in nature. When we encounter them, they risk increasing our experience of "lack" on some level. What is interesting, however, is that physical evil, for it to be a physical evil, has to interact with some metaphysical evil on some level. People drown in floods; ducks do not. It is a physical evil from the point of view of human beings, but ducks are largely indifferent to them, because they possess a nature better adapted to the overflowing water. Strictly speaking, any of the natural phenomena previously mentioned can actually be seen as something good — it all depends on your perspective! For a physical evil to really be considered a physical evil, it needs to coincide with some limitation present in the creatures with which it interacts, such that (due to that "metaphysical evil") the targets of the force in question have an increase in "lack" due to the interaction.
So if a loved one gets ill, the immediate question might be "where did the illness come from", but the existential question is really "why are we affected by illness at all?" Which then begs the question: what is God's plan to do something about this metaphysical evil?
Which brings us to the problem of moral evil.....
The created universe contains many non-rational things: clouds, rocks, trees, and so on. Their natures are limited, not only by what they are, but by their incapacity to grow in their "essence" (even if some external characteristics can change). But rational creatures are different: because they possess free will, they have the capacity to love, however imperfectly, and so they have the possibility to more perfectly participate in God's nature as the God-who-is-Love. And because the capacity for love can itself grow as it is exercised, it is possible for rational creatures to "grow in godliness" through the practice of genuine love, a process called "divinisation".
Now what is interesting about human beings is that we are composed of both matter and spirit. Our faculty for free will rests within our souls, but the body is also a true part of what makes us human. We therefore bridge the gap between the non-rational creation and God. It is a very early teaching of the Church that the first humans, who had not yet sinned, were called by God to act as a kind of "natural priesthood", "binding" creation to God such that God could more perfectly lead the non-rational things to a greater "being" through us. The expression the Bible uses to describe this is "God who is all in all".
Unfortunately, our earlier ancestors chose to "bind" the universe, not to God, but to Satan, the "opposer" of God. For this reason humans remained in our metaphysically weak creation; we lost the indwelling of the Holy Spirit; and we placed creation into a kind of bondage to the Evil One. Moral evil can truly be said to be the worst evil, because it prevented the curing of our metaphysical evil and therefore subjected us to physical evils, in some cases worse than what they might have been otherwise.
Now with regards to the Devil/Satan/Lucifer/whatever-his-name-is, where did he come from? Catholic tradition teaches that he was an angel, originally created good, but that he chose to become evil through his own choice. His sins are Pride and Envy. For Pride, he tried to become like God without God, by directing his love towards himself rather than to God (in effect, trying to become his own personal trinity). The disastrous consequence of this, for him, was the loss of the grace of the Holy Spirit, a metaphysical lack made permanent by the nature of the choice itself. As for Envy, this is a sin he directed to all other rational creatures (angels and human beings) who still retained the capacity to grow in godliness — looking upon this in others created the "pain in the heart" that defines envy, and he sought to ruin the capacity for love in these others.
This is the sorry state of affairs into which the world fell. Theoretically God could have just annihilated the universe and started over, but that would have simply increased the evil even further (as nothingness, as stated before, is actually the worst possible evil). So rather than re-create, God chose to redeem the world.
If God *is* all-good and all-powerful *and* there is evil in the world, what is God doing about it?
This third question is really the big one. Rather than go into detail regarding the Abraham, the prophets and the entire ministry of Jesus, let me just focus on the central element: the death and resurrection of Jesus Christ. This historical moment is at the heart of the work of Salvation. But how?
To understand Jesus' death, we need to understand what death is, and why it is so fearsome. When a living thing dies, it slides backwards on the scale of existence, because it now lacks something it had: life itself. Death is always an evil, then, because it is a lack of life for something that otherwise once was alive. For humans, though, death is even more dramatic, because we face the possibility of not only the loss of our biological life, but our inner rational life itself. Without any hope of some sort of afterlife, we face the extinction of the thing that really makes us human: the life of love that we have led. As much as animals shirk from death and seek to survive, the natural human horror of death therefore goes even further, as an attempt to reject the possibility of our very annihilation. But death still comes to us all, which means that the pall of despair hangs very heavily over much of human existence. Faced with such a bleak outlook, many choose to simply reject the value of leading a life of other-centred love in the first place (after all, what's the point if it's all going to disappear anyway?) and human existence becomes truly nasty and brutish.
Now with regards to Jesus, we need to keep in mind something very special about Him: that he is both perfectly God and perfectly human, "like us in all things but sin". In Jesus God became incarnate, emptying himself to truly take on our human nature. As such, he became subject in some manner to the limitations imposed by this limited human nature (a process called kenosis), including the law of death. But Jesus' death became the "death which trampled on death", by redeeming us and setting us free from the slavery of death.
How did it do this? We need to understand that Jesus' death was both willed by Satan and accepted by Jesus. Satan could not resist seeing Jesus killed, because (even if Satan didn't understand Jesus' true divinity) Jesus was clearly the Beloved of God — and Satan hates God (which explains the brutality of his death). Jesus, however, laid down his life as an act of perfect love, as he himself said: "Greater love has no man than this, that a man lay down his life for his friends." (John 15: 13) And it is true! To face what otherwise seems like annihilation of our very being, for sake of love, truly is a sign of the greatness of that love. And that love is so great that it makes us more into the image of God than ever before — so much so that it can overcome the very backsliding into nothingness that death otherwise represents.
In accepting to die for us, out of obedience to his Father, Jesus actually completed the "priestly" work of our first parents, breaking the chains that bound the physical world to Satan and re-attached them to God himself. This is why Jesus' death is called a "sacrifice", and New Testament refers to Jesus as the "great high priest". And while his body sat in the tomb, Jesus descended among the souls of the previously-dead to bring them the Good News that death itself was now conquered. For those who had not irrevocably damaged their souls through mortal sin, the possibility of entering into the communion of love of the Trinity now opened up.
Now while Jesus' death dealt with repairing the problems created by the failure of our first parents, setting the universe free from its bondage to Satan, there was still the problem of the metaphysical evil present in our basic human nature to deal with. The answer was the Resurrection. Jesus, during his ministry, brought a number of individuals back to life by his healing power, but they all eventually died again. Jesus, on the other hand, did not just rise in his body, but that body was also *glorified*, becoming the first piece of material creation to enjoy that perfect communion with God that overcomes all metaphysical evil. For example, Jesus ate with his disciples — but he didn't *need* to eat. It is a life of possibilities without limits.
Jesus promised to return again one day, and Christians are waiting in joyful hope for that great day, because Jesus' return is about a lot more than the mere geography of "where he is". When Jesus comes again all sources of deliberate evil (e.g. the evil angels) will finally be driven out of the universe, and the universe will be flooded with God's presence and power such that the universe will be "set free from its bondage to decay" and "God will be all in all". Jesus, therefore, is God's ultimate answer to the problem of evil, and he brings salvation to us in all its forms, moral and metaphysical.
If God *is* all-good and all-powerful *and* there is evil in the world *and* God is doing something about it, how do we get with the program?
The meaning of human existence, revealed in Jesus Christ, is to turn away from self to love God and love our neighbour in complete selflessness. Living in this way means growing in godliness even in this life, which fills our hearts with the hope that conquers despair. But how do we do this in practical terms?
The first critical element is to repent, i.e. to turn away from the things which God has revealed are sinful. There is no point in trying to attach ourselves to God if we keep weighing ourselves down with sin. But God knows that this is still a struggle for us, so many means of grace are made available to us to help us: the preaching of the Gospel, the encouragement of Christian fellowship, and (most importantly) the sacraments. I might also add that repentence is not just a one-shot moment of time (although it is sometimes given to us in particularly strong moments), but is a habit of mind and heart that continuously tries to avoid any nostalgia for past sin and which tries to dig out present sinful habits no matter how deep their roots. Repentence is a process, above all, of loosening the chains of sin that trap us.
Another critical element is to enter into the sacramental economy. The sacraments are amazing things: in them, God uses material elements as a means to communicate his grace to us. Baptism, the gateway to the other sacraments, cleanses us of sin and makes us into a new creation, with the indwelling of the Holy Spirit restored as per God's original plan for humanity. Confirmation builds us into the community of the Church, the Eucharist orients us towards Christ in a concrete way, Anointing of the Sick helps us face the reality of sickness and death with hope, the sacrament of Reconciliation helps us to concretely grow in humility and moral perfection, human erotic love is elevated to something spiritual through Marriage, and our need to be held together by some form of leadership is satisfied by having leaders graced with the sacrament of Holy Orders. When we are really and truly able to live the sacraments and their related liturgical elements, we are already getting a foretaste of the "heavenly liturgy" that will be celebrated in the universe once Jesus returns.
A final critical element is the undertaking of good works. Good works are not a means to "earn heaven", as nothing we can do can merit such an amazing gift. What they are, however, are a means to being into the world the very Love that is God. Love cannot be mere sentiment: it must lead to concrete loving acts, or else it is dead (cf. James 2). In performing such loving works, however, we not only grow in godliness ourselves, we also communicate God's love — indeed, his very nature — to the universe itself, foreshadowing that great day when the Beloved One sweeps up all of creation into the movement of love between himself and his Father