We Catholics often toss terminology around with the assumption that we all, of course, understand it and agree on its meaning. It is only when somebody (often it seems to be our children) asks "What do you mean by that?" that we find we don't really know what we mean.
One such commonly used, but little understood term, is "salvation." What do we mean by "salvation"?
Our very stammering at such a point-blank question shows something we don't mean. We do not mean salvation is "simple." Catholics are often embarrassed by this. We have grown submissive to bullies who like to attack us for taking the "simple message of Jesus" and (allegedly) "complicating it with dogmas, doctrines and theology." We need to shed that embarrassment. A basic fact about the universe faced by all grownups is that life is complicated. Therefore, a revelation which claims to help us navigate the enormous complexity of human existence is going to be complex too. To protest this is like demanding the science of cardiology be "simplified" or declaring that doctors have made the human heart a thing of impenetrable complexity in order to humbug the public with their priestcraft.
That said, we still need to have a sense of what salvation is. And a good definition is this: Salvation is complete union with the life of the Blessed Trinity. It begins with faith in Jesus Christ and baptism, but it does not end there.
This immediately distinguishes the Catholic understanding of salvation from the theology (though not the practice) of our Protestant brothers and sisters. Catholic and classic Protestant theology agree on this: we are saved by grace. There is no salvation, there is no act of love toward God or neighbor which was not inspired and supported entirely by God Himself. But Protestantism diverges from Catholic understanding when it concludes that we are therefore also saved by "faith alone". Catholics don't believe that faith alone saves us for they do not believe that an unincarnate faith can save.
We are not simply patients anesthetized on a table while the divine surgeon operates. Rather we are called to be involved in our salvation as well--by grace. That is why St. Paul elsewhere makes plain that faith alone-unincarnate, non-active, non-transforming faith-is not enough when he writes "If I have all faith so as to move mountains, but have not love, I am nothing" (1 Cor. 13:2). And this is repeated in both Sts. Paul and James. Both insist that faith is, like our Lord himself, inherently incarnational. It must be enfleshed, just as the Word was made flesh. It must act, just as God acts, or it is "dead" as St. James says.
When I was an Evangelical I often heard that we believed in salvation by grace alone but Catholics believed in salvation by grace plus works. The implication was that Catholics thought Jesus incompetent, as though they had to shore up his efforts with some good deeds since the poor man needed our help. Part of what prompted me to become Catholic was the discovery that, in fact, Catholic theology explained our behavior as Evangelicals better than Evangelical theology did. For we Evangelicals acted just like Catholics. We too believed that if you sowed to the Spirit, you would reap of the Spirit (as Paul said in Galatians 6:7). We too believed that we "grew in Christ" by obeying him. We too said that "faith is a muscle" and that you had to exercise it or it would atrophy. Seeing this, I began to see faith and works were no more divisible than the human and divine natures of Christ. I began to realize that, in my living experience, both Catholic and Evangelical live in an acting faith; a relational faith; a living faith and not a theological diagram. The only difference was, Catholic faith accounted better, not only for the biblical data, but also for the lived experience of all Christians, not just Catholics.
Salvation is, then, a living relationship with God. It is created and sustained by grace, born in faith, grows in love and bears fruit in glory. It not only forgives us, it changes us by our cooperation with grace. It makes us, in the words of St. Peter, "participants in the divine nature", not just forgiven sinners. Heady stuff, to be sure. But nobody ever said God is safe, only good.