The truth that established Churches and built them up cannot be improved up on. Paul preached the gospel. Christ died for the church. Paul preached unity 1 Cor. Paul did not preach unity in diversity.
Modern preachers attempt to preach unity by compromising the truth.
Paul did not preach or teach the future establishment of the kingdom of God, but told us how we can be in the kingdom now. We are to preach and teach what Paul taught.