Paul was teaching what Jesus Christ said. Paul was chosen by Jesus to replace Judas.
The teachings of Paul were accepted by all the other Disciples who were also chosen by Jesus.
2 Peter 3:14-16
[SUP]14 [/SUP] Therefore, beloved, looking forward to these things, be diligent to be found by Him in peace, without spot and blameless;
[SUP]15 [/SUP] and consider that the longsuffering of our Lord is salvation--as also our beloved brother Paul, according to the wisdom given to him, has written to you,
[SUP]16 [/SUP] as also in all his epistles, speaking in them of these things, in which are some things hard to understand, which untaught and unstable people twist to their own destruction, as they do also the rest of the Scriptures.
Even Peter said Paul was a Disciple who spoke for God.
You can reject everything Paul is saying, but even Peter accepted everything he said in his teachings.