Given the sin of impiety through which they [the Romans] sinned against the divine nature [by idolatry], the punishment that led them to sin against their own nature followed.... I say, therefore, that since they changed into lies [by idolatry] the truth about God, He brought them to ignominious passions, that is, to sins against nature; not that God led them to evil, but only that he abandoned them to evil.
Thomas AquinasIt is altogether unlawful to kill oneself... Wherefore suicide is contrary to the inclination of nature, and to charity whereby every man should love himself... Life is God's gift to man, and is subject to His power, Who kills and makes to live. Hence whoever takes his own life, sins against God... for it belongs to God alone to pronounce sentence of death and life.
Thomas AquinasBehold our refutation of the error. It is not based on documents of faith, but on the reasons and statements of the philosophers themselves. If then anyone there be who, boastfully taking pride in his supposed wisdom, wishes to challenge what we have written, let him not do it in some corner nor before children who are powerless to decide on such difficult matters. Let him reply openly if he dare. He shall find me there confronting him, and not only my negligible self, but many another whose study is truth.
Thomas Aquinas