Of Lawful Oaths and Vows, XXII:6

It is not to be made to any creature, but to God alone: and, that it may be accepted, it is to be made voluntarily, out of faith, and conscience of duty, in way of thankfulness for mercy received, or for the obtaining of what we want, whereby we more strictly bind ourselves to necessary duties: or, to other things, so far and so long as they may fitly conduce thereunto.

We make our oaths and vows to God alone, for only God has the knowledge and the power to ascertain the truth of our promises and to see justice done in the end. No vow is to be coerced--indeed, the best way to think of a vow is as a means toward doing that which we ought to do anyway.

Tomorrow: Unlawful oaths and vows.