People who knew much more than you included the letters of Paul in the Bible because they knew them to be true. Peter recognized the validity of Paul's teachings and their harmony with those of Christ.
Tyrone: More like people with an agenda included Paul's writings. And I'll not believe Peter endorsed the letters of Paul that are included in the bible. There is no indication that Peter ever read those screeds attributed to Paul. And 2nd Peter 3:15-16 are obvious additions to Peter's writing by a 3rd party.
Peter and Paul knew each other quite well. They spent a lot of time in the same city, Rome. They conversed often. What verse did you find where Peter condemns Paul?
That stuff about having an agenda is a real headscratcher. Exactly what agenda are you referring to?
The only agenda that I know of concerning Paul is that of the Pharisees and Sadducees undermining his gospel back then as many the Messianic Jews /Hebrew Roots Movement does these days. Paul was the outspoken zealot among the apostles IMHO. I think that the devil focussed on him more because Paul was so effective spreading the great commission.
Paul's gospel: Galatians 1 and 1 Corinthians 15:1-7
