Well, when I do good works certain spiritual benefits (merits) flow to me because of it. I grow in virtue and holiness for example, or lead others to Christ by example.
What the passage means is firstly that we only have the ability to do those works as a gift from God (we can do nothing good by ourselves). So it follows that the merits or spiritual benefits that flow from them are also gifts of God.
We don’t earn them by doing the good works, and cannot demand them as a reward in the same way that a hired worker who completes his assigned tasks earns and can demand his wages.
What does this passage have to do with your determination to reject Catholicism? I know some think mistakenly that the RC advocates salvation by works, but there’s not even a suggestion in that passage that it might. And, as I’ve stated, the contrary is true, as we do not earn salvation or any other spiritual good by our own works, but purely by God’s grace with both enables us to work and allows benefits to flow from it.
