Paul's authority is presumed. You assume that God exists; you assume that the Bible is God's own word; you assume that Paul is God's mouthpiece because he appears in the Bible.
You take his word that Christ spoke through him on faith and nothing but faith alone. That is your a priori acceptance of postulates; the rest simply fall in line by design, as I said in my earlier post. It is made to fit.
Once you accept that Paul is a mouthpiece of Christ, then everything he says is "true." Trouble is: Paul did not prove that he is a mouthpiece of Christ. We believe that he is. There is a big difference.
But that does not necessarly preclude that sense experience or even subjective states are not functional within a rationalist way of knowing
That's too broad of a generalization to mean anything, really.
One can know by a priori and still use sense experience and subjectivism to complement the a priori
The problem is with the a priori "knowledge." It is not knowledge; it is a belief, or an assumption. It is an anthropomorphic construct. In other words, it starts within us and is projected outward. It originates in us. Man-made.
Do YOU even believe in God? That Jesus is what the Bible says he is?