Most Protestants erroneously assume that Catholics believe in salvation by works. I could not begin to enumerate all the different Catholic works that demonstrate the falsehood of the notion, but here is a quick and short one from St. Thomas Aquinas, the Angelic Doctor:
We merit glory by an act of grace; but we do not merit grace by an act of nature. [ST I q.95 a.1 ad 6]
Short and sweet: we “merit” glory by grace. We could never earn or merit heaven on our own, nor by any power that we possess. It is a gift of grace. But we also do not merit grace: there is nothing that we can do by which we can pretend to say to God, “I deserve for You to give me the gift of heaven!” No. We are saved by grace from beginning to end.
Aquinas, Etc. 
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