Moralism has different meanings, but in relation to Christianity, it is the practice of keeping various moral laws so as to please God and obtain and/or maintain one’s salvation. Moralism is similar to legalism where keeping the Old Testament commandments is advocated in order to achieve and/or maintain salvation. On the other hand, there is a secular definition which is sometimes used in philosophical discussions:

“…moralism is often taken to denote a morality that has no links to God or to the orders of creation but that, as practical reason, defines good and evil on its own.”¹Fahlbusch, Erwin, and Geoffrey William Bromiley. The Encyclopedia of Christianity. Grand Rapids, MI; Leiden, Netherlands: Wm. B. Eerdmans; Brill, 1999–2003.

But again, as it relates to the Christian faith, moralism is the position of obtaining and/or maintaining our right standing with God by behaving morally. Such a teaching is a false gospel, which is warned about in Galatians 1:8-9. We are not made right before God by our moral behavior. We are not kept right with God by our moral behavior. Instead, our position with God is obtained completely by his grace. It is maintained by his grace and not by our adherence to moral principles and laws. After all, if righteousness could come through the law (keeping moral principles) then Christ died needlessly (Galatians 2:21).

Justification is the legal declaration of God upon the sinner where he declares the sinner righteous in his sight. In this, the righteousness of God is imputed to the believer (Philippians 3:9). This is achieved by faith alone in Christ.

• “For we maintain that a man is justified by faith apart from works of the Law,” (Romans 3:28).
• “But to the one who does not work, but believes in Him who justifies the ungodly, his faith is credited as righteousness,” (Romans 4:5).

Finally, if you want to get a better idea of what moralism is, then just think of the New Testament Pharisees who sought to do what was right in order to please God and be saved on the day of judgment. Jesus condemned this false gospel.

And He also told this parable to some people who trusted in themselves that they were righteous, and viewed others with contempt: 10 “Two men went up into the temple to pray, one a Pharisee and the other a tax collector. 11 The Pharisee stood and was praying this to himself: ‘God, I thank You that I am not like other people: swindlers, unjust, adulterers, or even like this tax collector. 12 I fast twice a week; I pay tithes of all that I get.’ 13 But the tax collector, standing some distance away, was even unwilling to lift up his eyes to heaven, but was beating his breast, saying, ‘God, be merciful to me, the sinner!’ 14 I tell you, this man went to his house justified rather than the other; for everyone who exalts himself will be humbled, but he who humbles himself will be exalted,” (Luke 18:9–14).