Protestants often point out the following:
For God so loved the world, that He gave his only begotten Son, that whosoever believeth in Him should not perish, but have everlasting life. --John 3:16 (KJV)
What do Catholics say to that? We say: “Amen!” like we say to everything in the Bible. But what does Jesus mean here by “believe”? Go down further in this chapter, and you will find out:
Whoever believes in the Son has eternal life, but whoever disobeys the Son will not see life, but the wrath of God remains upon him. --John 3:36 (KJV)
Disobeying God is not following his commandments. Both faith AND works (by God’s grace, not our own power) are necessary.
*“Yeah, but what about…" *
For by grace are ye saved through faith; and that not of yourselves; it is the gift of God: Not of works, lest any man should boast. --Ephesians 2:8,9 (KJV)
Keep in mind that St. Paul is speaking here, not about good works in the sense of fulfilling the Ten Commandments and avoiding evil, but about the works of the (Levitical) Law. Please read the entire chapter in context and you’ll see that it concerns St. Paul’s ongoing disputes with the “Judaizers” or “circumcision party” (see Acts 15). Same for the other famous “faith alone” proof-text, Romans 3:28.
Consider this: *If good works sprang up “naturally” out of faith, as some Protestants like to claim, almost the entire New Testament (which, recall, was addressed to “saved” believers) would be almost superfluous, since it **constantly **commands believers to do certain good actions and avoid certain evil ones. *
**It bears repeating: **Works are not the direct cause of our salvation; we aren’t saved by the works themselves, even those done as a response to and under the power of God’s grace.
But we ARE saved by works in the sense that if we don’t do the good works that God has set before us to do, or do evil works (mortal sins) in defiance of God’s clear prescription, this is disobedience and, as Jesus said in the passage above, the disobedient (including disobedient Christians) cannot be saved.
[God] will repay everyone according to his works: eternal life to those who seek glory, honor, and immortality through perseverance in good works, but wrath and fury to those who selfishly disobey the truth and obey wickedness. (Romans 2:6-8)
Another, secondary, way doing good works saves you is that when you occupy yourself with walking in God’s ways, you leave less room to fall into sins. When you head off even venial sin, it is harder for it to become habitual and lead you into serious sin:
No one experiencing temptation should say, “I am being tempted by God”; for God is not subject to temptation to evil, and he himself tempts no one. Rather, each person is tempted when he is lured and enticed by his own desire. Then desire conceives and brings forth sin, and when sin reaches maturity it gives birth to death. (James 1:13-15)
Occupy your minds with good thoughts, or the enemy will fill them with bad ones. Unoccupied, they cannot be. -St. Thomas More
Hope that helps.