Thomas Aquinas offers a persuasive argument in Summa Contra Gentiles I, 7:
“The truth that the human reason is naturally endowed to know cannot be opposed to the truth of the Christian faith. For that with which the human reason is naturally endowed is clearly most true; so much so, that is is impossible for us to think of such truths as false. Nor is it permissible to believe as false that which we hold by faith, since this is confirmed in a way that is so clearly divine. Since, therefore, only the false is opposed to the true, as is clearly evident from examination of their definitions, it is impossible that truth of faith should be opposed to those principles that the human reason knows naturally.”
“Therefore, we can say that either Christianity is false, or reason is false, or–if both are true — there can never be any real contradiction at all between them, since truth cannot contradict truth.” Kreeft, p.
“Furthermore, that which is introduced into the soul of the student by the teacher is contained in the knowledge of the teacher– unless his teaching is fictitious, which it is improper to say of God. Now the knowledge of the principles that are known to us naturally has been implanted in us by God; for God is the Author of our nature. These principles, therefore, are also contained by the divine Wisdom. Hence, whatever is opposed to them is opposed to the divine Wisdom and therefore cannot come from God. That which we hold by faith as divinely revealed, therefore, cannot be contrary to our natural knowledge.
“From this we evidently gather the following conclusions: whatever arguments are brought forward against the doctrines of faith are conclusions incorrectly derived from the first and self-evident principles embedded in nature. Such conclusions do not have the force of demonstration; they are arguments that are either probable or sophistical (fallacious). And so there exists the possibility to answer them.
Aquinas here is basically saying that any argument against Christianity doctrine has a rational mistake in it somewhere, and therefore can be answered by reason alone.
To this end, I am unsure if I agree with Aquinas. On the one hand, he seems to have a compelling argument as it simply follows from the first premise that if “Christianity is true”.. then “Christian irrationalism is false.”
One reason for my hesitancy is that it seems to imply that one can simply show all other worldviews against Christianity to be false by reason alone. Consequently, this seems to suggest that one can just do this to prove Christianity’s legitimacy or truth by reason alone.
Yet, on second thought this does not seem to follow, although it seems like it might be a temptation to some.
It must be clear that Aquinas is not claiming that all Christian Doctrines can be proved by reason alone only that every argument against them can be disproved.