Supposedly, Paul's words cannot be questioned, because he claimed to be speaking for God. He is his own witness.

But more basically, notice that Christ does not require anyone to base his words on him alone saying so. Instead, he shows the evidence. We can verify for ourselves whether his words are true by looking at the lessons of life. Christ taught through parables and examples including his own life, so we can relate everything he said to the lessons of life.

There is no other reason to teach. If it is not in the lessons of life, it is not learning. Getting a slogan without seeing it in life leaves no ability to determine what it means.

Yet Paul requires all of his words to be picked up as slogans with no evidence in life, and the simple reason is because he was teaching nonsense in conflict with reality.

So it is not valid to quote Paul as a proof for theology. Prove it through the lessons of life, as Christ did.

Paul