Do you doubt what is in scriptures written by Paul?
I don't. I believe all scripture is God-breathed, whether the words were written by Moses, David, Solomon, Matthew, Paul, or any of the other writers.
Also, Jaybird, here is what Peter, one of the original 12 who were with Jesus during his entire ministry, had to say about Paul's writings:
2 Pet 3:
15) And account that the longsuffering of our Lord is salvation; even as our beloved brother Paul also according to the wisdom given unto him hath written unto you;
16) As also in all his epistles, speaking in them of these things; in which are some things hard to be understood, which they that are unlearned and unstable wrest, as they do also the other scriptures, unto their own destruction.
It seems Peter believed what Paul wrote.