Catholics and Protestants agree that anyone who is saved is saved by God’s grace alone through faith, as a result of our Lord Jesus’ sacrificial and redeeming death on the cross for us, and that the elect who go to heaven are predestined by God to do so, and that (minus only Calvinists) we cooperate in our free will to accept this grace. 

It’s “both/and.” God does it, and we also “do” it by cooperating with him — “we are God’s fellow workers” (1 Corinthians 3:9); “I worked harder than any of them, though it was not I, but the grace of God which is with me” (1 Corinthians 15:10); “work out your own salvation with fear and trembling; for God is at work in you” (Philippians 2:12-13); “they went forth and preached everywhere, while the Lord worked with them” (Mark 16:20).

Furthermore, Catholics and Protestants agree that, after (initial) justification and regeneration, the Christian ought to do good works and make every effort to increase in righteousness and holiness — i.e., the process of sanctification — “as he who called you is holy, be holy yourselves in all your conduct” (1 Peter 1:15); “unless your righteousness exceeds that of the scribes and Pharisees, you will never enter the kingdom of heaven” (Matthew 5:20); “love one another as I have loved you” (John 15:12).

Luther and Calvin, the most prominent early Protestant leaders, technically separated sanctification from justification and salvation, but still strongly taught that good works were altogether necessary in the Christian life, as the manifestation or proof of a genuine, authentic faith. Protestants often state that “we are saved by faith alone, but not a faith that is alone.” 

We mustn’t caricature the beliefs of our Protestant brothers and sisters. Serious, theologically-educated Protestants do not teach antinomianism, or the notion that one is saved by faith alone; therefore they can live any sort of sinful, wanton lifestyle that they desire, without concern for eventual salvation. Our views are often caricatured and distorted; let’s not fall into the same error and sin in describing Protestantism.

Catholics and Protestants agree, for the most part, that virtually everyone will die with sin still present in their lives. They think it’s true of everyone; we believe it’s true of the vast majority (apart from a few saints who attain sinlessness).

Moreover, we agree that whoever gets to heaven will be free of actual sin — not merely declared to be so (as in Protestant imputed or forensic justification), per clear scriptural teaching. “Nothing unclean shall enter it” (Revelation 21:27); “the holiness without which no one will see the Lord” (Hebrews 12:14); “new heavens ... in which righteousness dwells” (2 Peter 3:13). Furthermore, several passages list sins that bar one from heaven (1 Corinthians 6:9-10; Galatians 5:19-21; Ephesians 5:5; Revelation 22:15).

The question then becomes: how does one become sinless and fit for heaven? Does this occur instantaneously (with God “zapping” us), as most Protestants believe, or is it more of a process? St. Paul seems to refer to purgatory itself: 

... the Day will disclose it, because it will be revealed with fire, and the fire will test what sort of work each one has done. ... If any man’s work is burned up, he will suffer loss, though he himself will be saved, but only as through fire. (1 Corinthians 3:13, 15)

The argument for purgatory as a process is mostly an analogical one, based on many passages concerning how God has already purified and refined believers during this life. St. Paul’s thorn in the flesh (2 Corinthians 12:7-9) is illustrative, which he says was given to him “to keep me from being too elated.” Then he recounts how God spoke and said, “My grace is sufficient for you, for my power is made perfect in weakness.” So Paul acquiesced and wrote, “I will all the more gladly boast of my weaknesses, that the power of Christ may rest upon me.” It’s very much like purgatory.

In Romans 5:3-5, Paul writes that “we rejoice in our sufferings, knowing that suffering produces endurance, and endurance produces character, and character produces hope, and hope does not disappoint us.” In the same epistle in 8:17, he proclaims that we are “fellow heirs with Christ, provided we suffer with him.” Three more key passages in this discussion are the following:

• For the Lord disciplines him whom he loves, and chastises every son whom he receives. It is for discipline that you have to endure. God is treating you as sons; ... For the moment all discipline seems painful rather than pleasant; later it yields the peaceful fruit of righteousness ... (Hebrews 12:6-7, 11)
• ...  for a little while you may have to suffer various trials, so that the genuineness of your faith, more precious than gold which though perishable is tested by fire, may redound to praise and glory and honor at the revelation of Jesus Christ. (1 Peter 1:6-7)
• Beloved, do not be surprised at the fiery ordeal which comes upon you to prove you, as though something strange were happening to you. But rejoice in so far as you share Christ’s sufferings, that you may also rejoice and be glad when his glory is revealed. (1 Peter 4:12-13)

We conclude, then, that purgatory — necessary to transform us in order to make us completely and actually holy in heaven — is a process, because we know that in its essential elements, the same purification and sanctification in this life take time, and there is no good reason to think that the methods that God uses in this life to make us more holy, should be fundamentally different in the next life.

We want everything right away (fast food, modern conveniences, etc.), but God’s timing and providence don’t work that way, as anyone who has seriously sought to live as a disciple of Jesus knows well from personal experience.