I grew up a committed legalist. Many who would make the same confession would then go on to blame their parents or the church of their childhood; but I do not see it that way.
Perhaps some of the early teaching I received contributed to the problem, but the problem was (and is) mostly internal. Legalism in one form or another is not second nature to fallen humans; it is first nature. It is deeply rooted in the way we think.
We want to justify ourselves. We want to save ourselves. As soon as we hear Paul saying, “For by grace you have been saved through faith. And this is not your own doing; it is the gift of God,” we want to start qualifying and watering down his statement. We fail to see that, in attempting to justify ourselves, we are robbing God of his rightful glory (see Romans 9:3-10:4), and saying that Christ died for no reason (Gal 2:21).
Do you ever wonder, “Am I a legalist?” If you do ask yourself that question, there is a chance that you are not a legalist. But, if you are real sure that you are not a legalist, then I would say it is nearly dead certain that you are. The litmus test for legalism, like so many things in Christianity, is counter-intuitive. The way up is down. The first is last; the last is first. A dying Savior gives us life. Those who think they see are blind; while those who know their own blindness have begun to see (Jn 9:40f). The person who recognizes his own tendency to self-justification has begun the cure. The person who does not recognize it in himself has not even begun to be cured.
Do not ask a drunk to explain drunkenness. He can’t. You have to be sober to understand drunkenness, wide awake to understand sleep, saved by grace to see the danger of legalism.
I hope that we are awake to our own legalism so that we can see its danger, flee from it, and cling to Christ.