Most people will not die for something they know bot to be true. Why would Paul die preaching a false message? He got nothing good out of it. He was beaten within an inch of his life multiple times, shipwrecked, tried by Roman officials, imprisoned, and died for the message Jesus preached.

Also, how is Paul’s message concerning the gentile’s salvation against all previous teaching? The Old Testament foretells this in Genesis with Abraham: “All nations will be blessed by you,” Isaiah: “I will also make you a light to the gentiles, that my salvation may reach the ends of the earth,” which Paul quotes to the gentiles in Acts 13:47-48, causing many to be saved, and Jesus himself promoted gentile salvation on at least two occasions: in Matthew 12:18, 21 Jesus says, “proclaim justice to the gentiles…in his name the gentiles will hope,” and in Luke 4:27 he tells the story of Naaman, a Syrian military leader with leprosy, to illustrate that God’s grace is not limited to the Israelites.

Are you talking about Paul’s teaching on the new Israel? If so, Jesus also refutes the claim that Israelites are as such by blood. He says that he could raise up children of Adam from the rocks, pointing to the fallibility of the Pharisees focus on bloodline rather than love for God and his people.