I don't know what this so-called pure gospel is. Was the preaching of John the Baptist pure? He did not say only, "Repent, for the kingdom of heaven is at hand!" (Matthew 3:2). He also "rebuked [Herod]...for all the evils which Herod had done" (Luke 3:19). He was beheaded because he didn't confine himself to abstract teaching. Jesus did not preach only the "pure" Sermon on the Mount, but also what some actual church leaders would have called a negative sermon: "Woe to you, scribes and Pharisees, hypocrites!...Serpents, brood of vipers!" (Matthew 23:27,33). It is for such "impure" preaching that He was crucified. The Pharisees would not have bothered about the Sermon on the Mount.