For I handed on to you as of first importance what I also received: that Christ died for our sins in accordance with the scriptures; 4that he was buried; that he was raised on the third day in accordance with the scriptures; 5that he appeared to Cephas, then to the Twelve. 6After that, he appeared to more than five hundred brothers at once, most of whom are still living, though some have fallen asleep. 7After that he appeared to James, then to all the apostles. 8Last of all, as to one born abnormally, he appeared to me.
But when he who had set me apart before I was born, and had called me through his grace, was pleased to reveal his Son to me, in order that I might preach him among the Gentiles, I did not confer with flesh and blood, nor did I go up to Jerusalem to those who were apostles before me, but I went away into Arabia; and again I returned to Damascus Gal 1:15-17
Paul believed he is a true witness of the 'Risen One' and has received his own mission directly from the Risen One. While Luke's account in Acts contains more embellished details, they agree that Jesus spoke to Paul, called him to the apostolate and made him a true Apostle, a witness of the Resurrection and the task of proclaiming the risen Christ to the Gentiles.