Why is Paul's gospel so different than Peter, James and John's?
Here are differences in preaching between the twelve apostles and the apostle Paul:
1) The name of Jesus
The twelve preached whosoever believed in the name of Jesus was saved (John 1:12 John 3:16-18 John 20:31 Acts 10:43). There was no mention of reconciliation by the cross (Colossians 1:20), and in fact the understanding of the death and resurrection was hid from the twelve (Luke 18:34).
Paul preached salvation in the name of Jesus (1 Corinthians 1:2 1 Corinthians 6:11 Ephesians 5:20) and by the cross (1 Corinthians 1:18 Colossians 1:20 Galatians 6:14 Ephesians 2:16).
2) Baptism
The twelve preached salvation upon baptism (Acts 2:38 Mark 16:16)
Paul was not sent to baptize (1 Corinthians 1:17)
3) The Holy Ghost
The twelve preached the receiving of the Holy Ghost upon baptism (Acts 2:38).
Paul preached that the Holy Ghost was received upon belief (Ephesians 1:13).
4) The kingdom
The twelve preached the kingdom gospel of the earthly Davidic kingdom (Luke 9:2 Luke 9:6).
Paul spoke of a heavenly kingdom (2 Timothy 4:18).
5) The resurrection
The twelve preached Jesus raised from the dead to sit upon the throne of David (Acts 2:30).
Paul preached that Jesus was raised for our justification unto eternal life (Romans 4:25).
6) Salvation
The twelve preached salvation at the coming of Christ (Acts 3:19 1 Peter 1:5 1 Peter 1:7).
Paul preached salvation now (Ephesians 2:8).
7) Forgiveness
The twelve preached forgiveness at the coming of Christ (Acts 3:19).
Paul preached forgiveness now in Christ (Colossians 1:14).
8) Destination
The twelve were appointed to judge the twelve tribes in the Davidic kingdom on earth (Luke 22:30). The twelve preached that believing Israel would be given the earthly kingdom (Luke 12:32).
Paul preached the heavenly places for the body of Christ (Ephesians 2:6).
Here are the similarities between the twelve and Paul:
1) The name of Jesus
The twelve preached the name of Jesus (John 1:12).
Paul preached the name of Jesus (Acts 9:29 Romans 1:5 1 Corinthians 1:2).
2) Jesus the Christ, the Son of God
The twelve preached Jesus as Christ, the Son of God (Matthew 16:16).
Paul preached Jesus as Christ, the Son of God (2 Corinthians 1:19).
3) The gospel of Christ
The twelve preached the gospel of Christ (Mark 1:1).
Paul preached the gospel of Christ (Romans 1:16) but with further revelations made known to him by revelations of the Lord Jesus Christ ( 2 Corinthians 12:7 Ephesians 3:2-7).
It can be decided upon comparing scripture as to whether or not the apostles all preached the same thing.