because to keep faith is of natural right, and a man cannot be commanded to do what is contrary to natural right.
 Thomas Aquinas, Summa Theologica (1274). copy citation

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Author Thomas Aquinas
Source Summa Theologica
Topic faith contrary
Date 1274
Language English
Reference
Note Translated by Fathers of the English Dominican Province
Weblink http://www.gutenberg.org/cache/epub/18755/pg18755-images.html

Context

“On the other hand sometimes they are such as one is not bound to make known, so that one may be under obligation not to do so on account of their being committed to one under secrecy. In such a case one is by no means bound to make them known, even if the superior should command; because to keep faith is of natural right, and a man cannot be commanded to do what is contrary to natural right. Reply Obj. 3: It is unbecoming for ministers of the altar to slay a man or to cooperate in his slaying, as stated above (Q. 64, A. 4) ; hence according to the order of justice they cannot be compelled to give evidence when a man is on trial for his life.” source