[lockeliberty]

How do we know that it is from God instead of from the adversary?

For the wrath of God is revealed from heaven against all ungodliness and wickedness of men who by their wickedness supress the truth. For what can be known about God is plain to them, because God has shown it to them. Ever since the creation of the world his invisible nature, namely, his eternal power and diety, has been clearly perceived in the things that have been made.

Even those who never had scripture had a knowledge of God. ie His moral law.

[wideawake]

And this is the Scriptural seat of the Catholic doctrine of right reason: that from examining the things that have been made (i.e. a posteriori deduction from creation) His invisible nature can be perceived.

St. Paul doesn't seem to be talking about solely a priori here, but rational discovery of natural law.