I came across a pretty good definition of the often bandied about term ‘LEGALISM’.  There is not a Greek word in our NT that we can strictly understand as meaning what we refer to as legalism.  So Paul invents different language to communicate this idea, like “works of the law”.

The erecting of specific requirements of conduct beyond the teaching of Scripture and making adherence to them the means by which a person is qualified for full participation in the local family of God, the church. — John Piper

This is where unbiblical exclusivism arises.  I left a church that admittedly and proudly practices a ’separatist’ philosophy when it comes to extra-Biblical matters.  There is no getting around the fact that the church does not include everyone. Peter refers to us as a “chosen race, a royal priesthood, a holy nation” ( I Peter 2:9).  But exclusion of people from the church should never be taken lightly. It is a very serious matter. Schools and clubs and societies can set up any human regulations they wish in order to keep certain people out and preserve by rule a particular atmosphere. But the church is not man’s institution. It belongs to Christ. He is the head of the body, and he alone should set the entrance requirements.

This is FAR from being an exhaustive look at this topic.  There are entire blogs devoted to the topic.  I’m what would be best described as a recovering legalist.  I’ve come out from it, but am too prone to fall back into my old ways.  There is some sick comfort in it.  A cheap replacement for the real life in Christ.