Many (or most, or bunches of) Protestants have the false idea in their heads that the Catholic Church teaches we are saved by our works: that we can merit entrance into heaven somehow based upon something we do. This is so common a mistake that it is practically assumed by lots of them to be an accurate thumbnail depiction of how things really are. Unfortunately someone hit that thumb with a hammer, and so the picture is completely distorted and swollen. And it hurts like heck.
Here is what the Compendium of the Catechism of the Catholic Church has to say about the subject:
In general merit refers to the right to recompense for a good deed. With regard to God, we of ourselves are not able to merit anything, having received everything freely from him. However, God gives us the possibility of acquiring merit through union with the love of Christ, who is the source of our merits before God. The merits for good works, therefore must be attributed in the first place to the grace of God and then to the free will of man. [§426; emphasis added]
No one, however, can merit the initial grace which is at the origin of conversion and justification. [ibid., §427]
Some critics might be tempted to suggest that these declarations reflect a spirit of modernity or modification at Vatican II, and that they do not accurately represent what the Church has always taught. These critics are mistaken. Take, for example, this statement from St. Thomas Aquinas:
Man can of his own accord fall into sin: but he cannot advance in merit without the Divine assistance. [ST I q.114 a.3 ad 3; emphasis added)
Some more stubborn critics might be disposed to claim that the Council of Trent abandoned what Aquinas taught. This too is mistaken, as is obvious from reading the Council’s Decree on Justification from the Sixth Session. For example, in the penultimate paragraph of chapter XVI we read:
far be it that a Christian man should either trust or glory in himself, and not in the Lord, whose goodness towards all men is so great, that He will have the things which are His own gifts to be their own merits. [emphasis added]
So there you have it. The Church has never taught that we are saved by our own merits, nor even that our own merits are, properly speaking, truly and completely our own. Rather, they are gifts from God. This comports with common sense. Natural powers (like any human activity whatsoever) can never bridge the gap to the supernatural—to God. It is not merely contrary to the Church’s teaching to say that we can earn salvation; it is also just plain irrational.
It might be worth mentioning that this view is consistent with the Bible, too. Look at what St. Paul says in 1 Thessalonians 2:19-20:
What do you think is our hope and our joy, and what our crown of honour in the presence of our Lord Jesus when he comes? You are, for you are our pride and joy. [NJB]
A crown of honour can surely only refer to some form of reward, and Paul says that the Thessalonians are his crown, undoubtedly because of their faithfulness. But this is not a reward that Paul could have earned himself, strictly speaking, because he was not able to give the Thessalonians the grace of God, nor the gift of faith. So if they are a reward, a crown for him, it can only be by the grace and gift of Christ and not by something done solely by Paul.
And similarly in 1 Corinthians 9:16-18:
In fact, preaching the gospel gives me nothing to boast of, for I am under compulsion and I should be in trouble if I failed to do it. If I did it on my own initiative I would deserve a reward; but if I do it under compulsion I am simply accepting a task entrusted to me. What reward do I have, then? That in my preaching I offer the gospel free of charge to avoid using the rights which the gospel allows me. [NJB]
In this passage St. Paul denies that he has earned any reward by preaching the gospel because he does so “under compulsion,” at God’s command. But he immediately proceeds to declare that he does expect a reward, though: a reward for preaching without compensation, which he could have rightly claimed from the churches he founded. In other words, St. Paul expects a reward not for preaching but for doing so without accepting payment from men. The worker is worthy of his wages, but Paul takes none. Again, though, this is not a reward he could earn on his own: God first called him to preach, and God made his preaching fruitful, and God moved him to preach without payment.