WCF 22.4.
An oath is to be taken in the plain and common sense of the words, without equivocation, or mental reservation. (Jer. 4:2, Ps. 24:4) It cannot oblige to sin; but in any thing not sinful, being taken, it binds to performance, although to a man’s own hurt. (1 Sam. 25:22, 32–34, Ps. 15:4) Nor is it to be violated, although made to heretics, or infidels. (Ezek. 17:16, 18–19, Josh. 9:18–19, 2 Sam. 21:1)