Now there should be a threefold order in man: one in relation to the rule of reason, in so far as all our actions and passions should be commensurate with the rule of reason: another order is in relation to the rule of the Divine Law, whereby man should be directed in all things: and if man were by nature a solitary animal, this twofold order would suffice. But since man is naturally a civic and social animal, as is proved in Polit. i, 2, hence a third order is necessary, whereby man is directed in relation to other men among whom he has to dwell (ST I-II Q72 A4).Aquinas can say that all our actions should be "commensurate with the rule of reason" precisely because the Christian faith is not contrary to reason, even when it exceeds the capacity thereof. But reason is not the sole order of our actions, and it's certainly not the primary rule, which is the Divine Law. Above all we ought to order our lives according to this Law, because it is the Law of our Creator and Redeemer. But it is also reasonable for us to do so:
Of these orders the second contains the first and surpasses it. For whatever things are comprised under the order of reason, are comprised under the order of God Himself. Yet some things are comprised under the order of God, which surpass the human reason, such as matters of faith, and things due to God alone (ibid).Not only do some things surpass human reason, but the difficulty of reasoning rightly makes the Divine Law superior as well, as St. Thomas has said elsewhere (and I am having trouble putting my finger on it at the moment, so no link. Sorry): because it is hard, because men have varying gifts with respect to the ability to reason, because they likewise have differing opportunities for doing so due to their circumstances, reason by itself is an insufficient guide with respect to divine things - in addition to the fact that they by their nature exceed reason's measure.
But then St. Thomas adds that there is a third order as well - and that is social. It is part of our nature, as God said in creating Eve: "It is not good for the man to be alone." It's not just that we need the company of women, but that we need company. We are social beings. We don't live in communities just because of some social contract, but rather we do so by nature, because we are human.