Legalism is the insistence on judging spirituality on the basis of keeping moral laws -- usually a particular group of moral laws elevated to "gatekeeper" status for acceptance or membership in a group. Over time I've seen a number of different "gatekeeper" lists of rules. It's tempting to break them down into man-made rules (against smoking, alcohol, caffeine, dancing, card-playing) versus God-made rules (against murder, adultery, lying, stealing). Oddly, the religious groups with "gatekeeper" rules often emphasize the man-made rules, perhaps thinking that the God-made rules aren't very distinctive or don't genuinely guarantee a holy lifestyle. But the reason I call legalism morally bankrupt goes beyond the emphasis on man-made rules to the de facto unseating of the greatest laws: to love God and neighbor.
Legalism is an attempt to control sin and to justify people who keep the list -- but "the list" never seems to have God's own priorities at the top, so it never works on either count. It troubles tender consciences over laws that are frequently man-made, while offering false security to an angry or loveless soul that manages to toe the line. Any satisfaction it gives is misleading at best.
There are different kinds of people in the world. Some love God. Others love to be in control. Both types may be drawn to religion, but for very different reasons. It becomes a problem when the "love control" group actually succeeds in being in control within a religious setting. This is an inherent risk since the "love God" group does not primarily seek control. This was a problem in Jesus' day to the extent that his main opponents were the "most religious" of the day.
The anti-Christian crowd has it wrong in imagining that it is Christianity's aim to force others to bow to its will. But we may miss the fact that people can use religion to legitimize their own overly controlling tendencies, and that such people are sometimes drawn to religion for the opportunities it affords. When we do not take an honest look at the problem, we allow it to happen more often than it should. At the worst of times it becomes nearly a co-dependent situation. The "love God" crowd may want someone else to tell them what to do about their love of God while at the same time (forgive me) having a ready-made excuse for being a bit lazy about leadership. The "love control" crowd gets to tell the "love God" crowd what to do and have all the appearance of holiness whether or not they kindle their love of God and neighbor.
What to do about it? Churches should seek to develop leaderhip amongst people who plainly love God and neighbor and who lead well as evidenced by order in their own families. If you have graciously passed up several invitations to serve at your church -- not being interested in control -- please accept the next invitation to serve. If you have put yourself forward for numerous opportunities to take things in hand, ask yourself whether this has taken a toll on your love of God and neighbor, and let go of the excess in order to take control of one more thing: the time to renew yourself.