Why was Paul's gospel at Antioch completely different from everything that had come before?
Paul's gospel at Antioch differed because it was a secret hidden in God's mind until revealed, something God was now doing totally different. Unlike Jewish believers under the Judaistic umbrella who only believed Jesus was Israel's Messiah, this gospel declared He died for them, shed His blood for sin, and was raised from the dead.
Les explains that Barnabas brought Saul to Antioch, where for a whole year they assembled with the church and taught much people. The disciples there—primarily Gentiles—were first called Christians at Antioch, not Jerusalem, even though the Jerusalem church had continued for twelve years. Les notes the Bible never calls the Jewish believers at Jerusalem and Judea Christians; they were under the Judaistic umbrella and only believed Jesus was the Messiah of Israel. But these Christians believed He had died for them, His blood had been shed for their sin, and He had been raised from the dead. Antioch was thus being presented with Paul's Gospel, which he differentiated from everything before because it was a secret held in the mind of God until it was revealed that God would now do something totally different.