Why does Paul speak against legalism?
Paul opposes legalism in any shape or form because believers are not under the Law but under Grace. Law and Grace are two contrary principles that will not mix, so legalism contradicts the grace-based relationship God has established for us today.
Les explains that Paul consistently contrasted the wisdom of men with the wisdom of God, and the Corinthians remained carnal rather than feasting on the wisdom available to them. The same applies today: every believer can study the Book and feast on it, but most will not. Les's first premise is to separate what is for the Jew from what is for the Gentile. This does not mean discarding the Jewish portion, but comparing Scripture with Scripture. God dealt with the Jew under the Law, while He deals with us today under Grace. Law and Grace are two totally contrary principles that will not mix. Therefore Paul cries against legalism in any shape or form, because believers are not under the Law but under Grace.