Antinomianism and legalism always feed each other in a vicious negative spiral. Antinomista (to use the Marrow personalities) says, "The law doesn't bind me, and therefore I can do what I want." Nomista says, "But look at where that leads you. At least I don't have YOUR problems! I'm much closer to heaven than you are!" To which Antinomista says, "You're just too straight-laced and have no grace in your heart." And so, on and on. You're nearly always going to find the other where you find one of them, because that is simply how human nature responds. Only the full gospel with defined, distinguished, and inseparable benefits located in Christ Himself will answer or curtail the negative spiral. I have seen plenty Antinomistas and Nomistas, in about equal measure.
Antinomianism was one of the main boogeymen of the Westminster Assembly. I am so thankful that the Assembly's response goes back to solid gospel bedrock instead of trying to balance out the rejection of the law solely with law-preaching. So many legalists think that the answer to antinomianism is law. The reject the law. Therefore we should emphasize law (even at the expense of gospel!). The answer to antinomianism is the gospel of justification AND sanctification in Jesus Christ (and yes, the law plays a very important role in showing us our need of Christ, and way we must walk as Christians in sanctification). The full-orbed gospel of forgiveness AND renovation is the gospel answer.
Legalism is harder to spot, because it is less obvious, and there are many more forms of it. People tend to think of legalism as less prevalent. I think that is mostly because of its hiddenness. Legalism is the person saying, "I'm ok because I go to church and serve people there." They are the people who rightly think works are necessary to salvation (defined broadly as including all the benefits of salvation), but fail to realize that the works are only necessary in a posterior and resultative way (like the sound of a cannon is necessary to the explosion of a cannonball being fired, my favorite illustration: anyone ever heard of a cannon silencer?? The sound obviously does not cause the cannon to go off, but is instead the necessary, posterior, and resultative effect of the explosion), not an antecedent and causative way. Thus says Turretin, basically.
Here are some variants of legalism that are quite pernicious, but much more sly than crass works salvation. 1. Man-made church rules are just as important (de facto) as God's law. 2. What I do as a Christian keeps me in the fold and ok. 3. Sin proves that a person is not a Christian. 4. Grace shouldn't be shown to people who have blown it. 5. We need to "flesh out" the law of God to address every possible specific situation, and the fleshing out is the same thing as God's law. 6. My experience with God's law is determinative for everyone else. I think if we start to think more thoroughly about legalism, we will realize that it is just as prevalent as antinomianism.
One last point to make: legalism and antinomianism, ironically, both devalue God's law. Legalism believes that the law is keepable for fallen humans (thus diminishing its requirements), while antinomianism believes keeping the law is irrelevant. They are both opposed to the gospel, and are equally pernicious.
