But on two accounts it is difficult to proceed against each particular error: first, because the sacrilegious utterances of our various erring opponents are not so well known to us as to enable us to find reasons, drawn from their own words, for the confutation of their errors: for such was the method of the ancient doctors in confuting the errors of the Gentiles, whose tenets they were readily able to know, having either been Gentiles themselves, or at least having lived among Gentiles and been instructed in their doctrines. Secondly, because some of them, as Mohammedans and Pagans, do not agree with us in recognising the authority of any scripture, available for their conviction, as we can argue against the Jews from the Old Testament, and against heretics from the New. But these receive neither: hence it is necessary to have recourse to natural reason, which all are obliged to assent to. But in the things of God natural reason is often at a loss.
1.1 : The Function of the Wise Man
1.3 : That the Truths which we confess concerning God fall under two Modes or Categories