Man can sin against nature in two ways. First, when he sins against his specific rational nature, acting contrary to reason. In this sense, we can say that every sin is a sin against man's nature, because it is against man's right reason.
Thomas AquinasIt is necessary to posit something which is necessary of itself, and has no cause of its necessity outside of itself but is the cause of necessity in other things. And all people call this thing God.
Thomas AquinasTo one who has faith, no explanation is necessary. To one without faith, no explanation is possible.
Thomas AquinasCommemoration of Gilbert of Sempringham, Founder of the Gilbertine Order, 1189 Some there are who presume so far on their wits that they think themselves capable of measuring the whole nature of things by their intellect, in that they esteem all things true which they see, and false which they see not. Accordingly, in order that man's mind might be freed from this presumption, and seek the truth humbly, it was necessary that certain things far surpassing his intellect should be proposed to man by God.
Thomas Aquinas