A lawful oath is an act of religious worship, in which the person swearing in truth, righteousness, and judgment, solemnly calls God to witness what he swears, and to judge him according to the truth or falsity of it.
Only by the name of God can a righteous oath be sworn, and only if it is used with the utmost fear of God and reverence. Therefore, to swear vainly or rashly by the glorious and awesome name of God, or to swear by any other name or thing, is sinful, and to be regarded with disgust and detestation. But in matters of weight and moment, for the confirmation of truth, and for the ending of strife, an oath is sanctioned by the word of God. Therefore a lawful oath being imposed by a lawful authority can rightly be taken in such circumstances.
Whoever takes an oath sanctioned by the word of God is bound to consider the weightiness of so solemn an act, and affirm or confess to nothing except that which he knows to be true. For by rash, false, and vain oaths, the Lord is provoked and because of them this land mourns.
An oath is to be taken in the plain and common sense of the words without equivocation or mental reservation.
A vow, which is not to be made to any creature but to God alone, is to be made and performed with all the utmost care and faithfulness. But monastic vows (as in the Church of Rome) of a perpetual single life, professed poverty, and regular obedience, so far from being degrees of higher perfection, are superstitious and sinful snares, in which no Christian may entangle himself.