Why would Paul be a liar if he were just another apostle?
Paul would be a liar because he consistently claimed in his letters that God revealed the great doctrines of grace to him and him alone. If he were merely another apostle, that exclusive revelation claim would be false, contradicting his own defense of apostleship.
Les explains that after dealing with Israel for three chapters, Paul addresses Gentile believers, saying, 'I speak to you Gentiles, inasmuch as I am the apostle of the Gentiles.' Some newer translations change 'the' to 'an,' which Les says violates the text and changes Paul's whole position. If Paul were just another apostle, he would be a liar, because he claims in all his letters that it was to him and him alone that God revealed these great doctrines of grace. Paul also said, 'I magnify my office,' meaning he would never back down from being God's man for the Gentile world.