What two things is Paul comparing in 1 Corinthians 3?
Paul compares the wisdom of men with the wisdom of God. The Corinthian believers remained carnal, failing to access the fountain of wisdom available to them. This contrast runs through 1 Corinthians 3, showing that human wisdom is opposed to God's wisdom, which believers can receive by feasting on Scripture.
Les explains that Paul has been constantly comparing the wisdom of men with the wisdom of God, yet these Corinthians are still so carnal that they are not cashing in on this fountain of wisdom that is theirs for the taking. It is the same way for us today: every believer can simply get into the Book and feast on it, but most won't. The excuse "I can't understand it" is not valid, because it is not that difficult. Les's first premise is to separate that which is for the Jew from that which is for the Gentile, comparing Scripture with Scripture, in light of the fact that God dealt with the Jew under the Law while dealing with us today under Grace. Those are two totally contrary principles; Law and Grace will not mix, and Paul cries against legalism in any shape or form because we are not under the Law but under Grace.