why is Paul so against legalism?
Paul opposes legalism because believers are not under the Law but under Grace. Law and Grace are two contrary principles that will not mix. God dealt with the Jew under the Law, but deals with us today under Grace, so legalism in any form contradicts the believer's standing.
Les explains that Paul constantly compares the wisdom of men with the wisdom of God, and the Corinthians were still so carnal that they were not cashing in on the fountain of wisdom available to them. Les's first premise is to separate what is for the Jew and what is for the Gentile, not throwing aside the Jewish portions but comparing Scripture with Scripture. God dealt with the Jew under the Law, but He is dealing with us today under Grace. These are two totally contrary principles; Law and Grace will not mix. Therefore Paul cries against legalism in any shape or form, because we are not under the Law but under Grace. Every believer can get into the Book and feast on it, but most won't, which is the same carnality.