Legalism is best disguised when it takes up residence in our consciences. From there it can taunt us, urging us to do better and try harder in our feeble fallen existence.
Such guilt-riddled consciences long to be soothed. Invariably, false religion steps into that void, offering a system of works. Man-made religions are particularly appealing to burdened sinners desperate to silence the cries of their consciences.
Roman Catholicism is a great example. We’ve already pointed out their codified denials of salvation by grace alone. On top of that, however, Catholic dogma also affirms works-righteousness through their doctrine of penance:
Raised up from sin, the sinner must still recover his full spiritual health by doing something more to make amends for the sin: he must “make satisfaction for” or “expiate” his sins. This satisfaction is also called “penance.” Catechism of the Catholic Church, Paragraph 1459.
In his book, The Gospel According to Rome, James McCarthy explains how penance is implemented and enforced among Roman Catholics: “To assist the person in making reparation for his sin, the priest imposes an act of penance. It is selected to be ‘in keeping with the nature of the crimes and the ability of the penitents.’”  James G. McCarthy, The Gospel According to Rome (Eugene, OR: Harvest House Publishers, 1995), 79. Prior to his conversion, Martin Luther was deemed by his Catholic peers to be a penitent with a lot of “ability.” As a result, he suffered egregiously under the weight of Roman Catholic penance.
Luther vs. Legalism
Luther’s conscience was plagued by his inability to conquer the sin in his life. Thus he was constantly putting himself through rigorous penance requirements, as James Kittelson vividly describes:
Long periods with neither food nor drink, nights without sleep, bone-chilling cold with neither coat nor blanket to warm him—and self-flagellation—were common and even expected in the lives of serious monks. . . . [Luther] did not simply go through the motions of prayers, fasts, deprivations, and mortifications of the flesh, but pursued them earnestly. . . . It is even possible that the illnesses which troubled him so much in his later years developed as a result of his strict denial of his own bodily needs.  James M. Kittelson, Luther the Reformer, Fortress Press ed. (Minneapolis, MN: Fortress Press, 2003), 55.
It’s little wonder that Luther’s conversion to Christ was intensely euphoric and liberating. The apostle Paul’s words in Romans 1:17—“the Righteous man shall live by faith”—were the lightning rod that ignited Luther, sparked the Reformation, and shook the world.
Professing Protestants, Practicing Catholics
All true Protestants gladly join with Martin Luther in proclaiming the righteousness that comes through faith in Christ—completely apart from any human effort. Even so, the harsh practices of his former monastery often linger in our cloistered consciences.
To be sure, most Christians find the Catholic doctrine of penance to be abhorrent. Nonetheless, many are inwardly self-flagellated by their guilt-riddled consciences. They know their right standing with God hinges on Christ’s atoning work, yet still consider it to be a fragile reconciliation—one that’s on a perpetual knife edge. God may have adopted them as His children, but they still live in constant fear of being disowned if they commit a big enough sin. For this reason, many churches are full of Protestants who think and act like Catholics.
I should know, because I used to be one of them. I understood that I was saved by grace alone through faith alone in Christ alone. But my relationship with God felt like a continual roller coaster that went up and down with my behavior. Some days I felt extra obedient to His commands and consequently walked with confidence that God must be pleased with me. Other days I felt humiliated by acts of disobedience and was too embarrassed to approach Him in prayer. Then it was up to me to tip the scales back in my favor by trying harder and doing better.
It may have been imperceptible to my Christian friends, but my mind was racked with legalistic guilt and fear. Worse still, I actually thought my mental penance demonstrated great humility and righteousness. But living under that kind of pressure isn’t a form of piety, nor does it reflect a low view of self. Rather, it reveals unbelief concerning God’s Word and a low view of Christ in His role as our heavenly Advocate:
If we confess our sins, He is faithful and righteous to forgive us our sins and to cleanse us from all unrighteousness. If we say that we have not sinned, we make Him a liar and His word is not in us. . . . And if anyone sins, we have an Advocate with the Father, Jesus Christ the righteous; and He Himself is the propitiation for our sins. (1 John 1:9–2:2)
Concerning Christ as our Advocate, John MacArthur writes:
All those standing before the bar of divine justice are guilty of violating God’s holy law; they “are all under sin; as it is written, ‘There is none righteous, not even one’” (Romans 3:9–10), and “whoever keeps the whole law and yet stumbles in one point, he has become guilty of all” (James 2:10). The just sentence the divine court should hand down is eternal punishment in hell, “for the wages of sin is death” (Romans 6:23).
But all is not hopeless for the guilty, because there is one more character to consider in this divine courtroom scene: the Lord Jesus Christ. He acts as the Advocate, or Defense Attorney, for all those who believe savingly in Him. He is a most unusual defense attorney, however, since He does not maintain His clients’ innocence, but rather acknowledges their guilt. Nonetheless, He has never lost a case—and never will (John 6:39; cf. Romans 8:29–30). Using the language of the courtroom, Paul declared, “Who will bring a charge against God’s elect? God is the one who justifies; who is the one who condemns? Christ Jesus is He who died, yes, rather who was raised, who is at the right hand of God, who also intercedes for us” (Romans 8:33–34; cf. Colossians 2:13–14). That last phrase is the key to how the Lord Jesus Christ infallibly wins acquittal for those who put their faith in Him. He intercedes with the Father on the basis of His own substitution for sinners in sacrificial death, which fully paid sin’s penalty for all who trust Him for salvation, thus meeting the demands of God’s justice.  John MacArthur, The MacArthur New Testament Commentary: 1–3 John (Chicago, IL: Moody Publishers, 2007), 44.
We Depend on Christ to Save Us and Keep Us
It was John MacArthur who delivered the death blow to my inner legalism when he said, “If I could lose my salvation, I would.” I immediately understood his point. If maintaining my right standing with God hinged on my own efforts at pleasing Him, then it would be as doomed to failure as any self-effort to bring about my own salvation. We must depend on Christ for everything. If I am trusting Him to save me then I also need to trust Him to keep me from ever falling out of His grace.
I give eternal life to [My sheep], and they will never perish; and no one will snatch them out of My hand. My Father, who has given them to me, is greater than all; and no one is able to snatch them out of the Father’s hand. (John 10:28–29)