There exist two thieves of the Gospel. Wherever the Gospel is present, these two thieves stand on either side of it. Every Christian is at risk of leaning toward one or the other. In fact, some Christians swing dangerously back and forth like a pendulum between the two. Some Christians will even pick and choose which of the two they will apply to certain areas of their life, like the religious hypocrites they are. These two thieves are Legalism and Antinomianism. Timothy Keller has said, "Antinomianism steals from the Gospel by twisting the grace of God into a license to do whatever you want while legalism steals from the Gospel by twisting the edginess of grace into law. And both look for ways to get out from under the shadow of the cross. Both try to avoid Jesus as Lord and Savior. Antinomianism avoids Jesus by resisting His lordship. Legalism avoids Jesus by rejecting His saviorship."
Antinomianism: Paul addressed this issue in Romans when he asked, "What shall we say then? Are we to continue in sin that grace may abound? By no means!" (Rom. 6:1-2). The meaning of this word is very simple. Anti means "against" and nomianism refers to a particular approach to the law. Simply put, Antinomianism means "against the law" or "no law." This term is used by theologians to refer to Christians who like to turn grace into a license to sin, or licentiousness. Licentiousness is Antinomianism. Jude addressed false teachers "who turn the grace of our God into licentiousness" (v. 4). Antinomianism looks at how free grace is and then twists it to mean that our thoughts, attitudes, and behaviours—the way we live—do not really matter to God because we are forgiven. Antinomianism supposes that sin is not that big of a deal.
Legalism: Paul addressed this issue is Colossians when he said that people make rules "according to human precepts and teachings," and that these rules only have "an appearance of wisdom" while being "of no value in stopping the indulgence of the flesh" (Col. 2:21-23). Legalism is the exact opposite of Antinomianism. Legalists assume that the Christian faith is performance-based. Many Christians are consumed with outward conformity to rules while at the same time not being concerned with sin in their hearts. Legalism says we are accepted by God based on our own accomplishments. It says we can live up to the standards that God has set. Legalism has "an appearance of godliness" but is guilty of "denying the power thereof" (2 Tim. 3:5).
While grace is free and gives us freedom in Christ, there is still an edge to grace. Grace never says we are completely and utterly free to say or do whatever we want, whenever we want, however we want. Legalism acknowledges that there is an edge to grace, but goes about dealing with that edge in all the wrong ways. Antinomianism forgets that there is an edge to grace, and thinks it does not matter if we sin or even how we sin. It presumes on the forgiveness of God. The Christian needs to be mindful of both of these thieves of the Gospel and to speak the Gospel to him/herself daily.