Sin is a word, deed, or desire, contrary to the eternal law (ST I-II Q71 A6).St. Thomas is not the only one to have followed St. Augustine on this score. The CCC (1849) quotes them both on this subject. And here is what the Westminster Shorter Catechism (considered a standard of faith by conservative Presbyterians and others) says:
Q. 14. What is sin?But It is not enough, as Aquinas says in response to objection 5 for this article in the Summa, that sin might be better defined as being contrary to reason. While it is certainly true that many sins are contrary to reason, this is an insufficient standard.
A. Sin is any want of conformity unto, or transgression of, the law of God.
The theologian considers sin chiefly as an offense against God; and the moral philosopher, as something contrary to reason. Hence Augustine defines sin with reference to its being "contrary to the eternal law," more fittingly than with reference to its being contrary to reason; the more so, as the eternal law directs us in many things that surpass human reason, e.g. in matters of faith (ST, ibid.; ad 5).The Christian faith is not rationalistic. It cannot be circumnavigated by reason, and it cannot be fully comprehended by reason either. But it is possible for a man to sin in regard to matters of faith - as when a Catholic denies a dogma of the Church such as the Trinity, which cannot be known by reason. Therefore it would be unreasonable to limit sin to those matters which are comprehensible by reason.