In the book of Galatians, the apostle Paul fervently argues against a group of legalistic teachers who were causing trouble. One of the first heresies in the early church was that of believing that a non-Jew had to adopt Jewish customs to be a proper Christian.
For instance, they thought that the Gentile Christians needed to be circumcised like the Jewish Christians were. The book of Galatians is the strongest work in the Bible against this legalistic false teaching, and against legalism in general.
Legalism is believing that there are other works that one must do in addition to having faith in Christ in order to be a Christian. Because of it’s emphasis on self-righteous works, legalistic people tend to be boastful. In Luke 18, the Lord Jesus described a Pharisee, who are infamous for their legalism:
11 The Pharisee stood and prayed thus with himself, God, I thank thee, that I am not as other men are, extortioners, unjust, adulterers, or even as this publican. 12 I fast twice in the week, I give tithes of all that I possess. (Luke 18:11–12)
Let’s call this boasting…