Protestants often think that Catholics believe that one can earn their salvation

Protestants often think that Catholics believe that one can earn their salvation. This is exactly the opposite of what the Church teaches. The Council of Trent stressed: "[N]one of those things which precede justification, whether faith or works, merit the grace of justification; for if it is by grace, it is not now by works; otherwise, as the Apostle [Paul] says, grace is no more grace" (Decree on Justification 8, citing Rom. 11:6).

The problem arises with the term “merit” and the American understanding that this word means “earning as in the strict sense” but in the second century the Latin word for merit was introduced as a synonym for the Greek word for ‘reward” thus merit and reward are one in the same.(The Fathers Know Best, Jimmy Akin)

Paul tells us: "For [God] will reward every man according to his works: to those who by perseverance in working good seek for glory and honor and immortality, he will give eternal
life. There will be . . . glory and honor and peace for everyone who does good, the Jew first and also the Greek. For God shows no partiality" (Rom. 2:6–11; cf. Gal. 6:6–10).

The Catholic Church teaches only Christ is capable of meriting in the strict sense—mere man cannot “With regard to God, there is no strict right to any merit on the part of man. Between God and us there is an immeasurable inequality, for we have received everything from him, our Creator.” (CCC 2007). The most merit humans can have is condign—when, under the impetus of God’s grace, they perform acts which please him and which he has promised to reward “The merit of man before God in the Christian life arises from the fact that God has freely chosen to associate man with the work of his grace. The fatherly action of God is first on his own initiative, and then follows man's free acting through his collaboration, so that the merit of good works is to be attributed in the first place to the grace of God, then to the faithful. Man's merit, moreover, itself is due to God, for his good actions proceed in Christ” (CCC 2008).