What are Lawful Oaths and Vows?
A lawful oath is an act of worship in which the person swearing in [the light of God's] truth, righteousness and judgement, solemnly calls God to witness what he swears, and to judge him according to the truth or falsity of it.
People should swear by the name of God alone, and His name is to be used with the utmost holy fear and reverence. To swear vainly or rashly by that glorious and awesome name, or to swear at all by any other thing, is sinful and to be abhorred. An oath is sanctioned by the Word of God in weighty and momentous matters to confirm truth and end strife, so a lawful oath imposed by lawful authority ought in such circumstances to be taken.
Whoever takes an oath sanctioned by the Word of God ought to consider the seriousness of so solemn an act, and to affirm nothing but what one knows to be the truth. For by rash, false and empty oaths the Lord is provoked and because of them a nation is brought to misery.
An oath is to be taken in the plain and ordinary sense of the words, without ambiguity or mental reservation.
A vow is not to be made to any creature, but to God alone, and it is to be performed with the utmost care and faithfulness. But monastic vows of a perpetual single life, professed poverty, and regular obedience, are far from representing degrees of higher perfection, rather, they are superstitious and sinful snares in which no Christian ought to entangle himself.
 Hebrews 6:13-16; Genesis 24:3; 47:30-31; 50:25; 1 Kings 17:1; Nehemiah 13:25; 5:12; Ezra 10:5; Numbers 5:19,21; 1 Kings 8:31; Exodus 22:11; Isaiah 45:23; 65:16; Matthew 26:62-64; Romans 1:9; 2 Corinthians 1:23; Acts 18:18