Paul was quite literally God's messenger.

He was separated from the others by God for a specific purpose.

No matter who tried to change his belief, whether it was the other Apostles earlier, or man through either persuasion or torture, Paul never wavered in his belief that the Evangel that he was extolling to any and all was the only true gospel.

Why was he so fixed in his belief?

Because he was taught the Evangel by the Spirit of God, alternately the Spirit of  Jesus Christ. 

He continually reminds us that he never received teaching from "man".  Everything that concerned God and His plan was obtained at the "foot" of the heavenly Spirit.

So, take a page from Paul and remember that, now that you believe in Paul's teaching/evangel, let no "man" change your belief.

There may be times when you will question whether it's sufficent for you to just "believe", but then  remember that You have received the truth from God's Messenger..paulnbob