"The Third Commandment is the only commandment that gets two Lord's Days instead of one. And oddly enough, the extra Lord's Day for the Third Commandment deals with an issue that seems rather prosaic. With a hundred other pressing ethical dilemnas to choose from, frankly, the swearing of oaths seems like a pretty poor choice. After all, how often have you heard about Christians reading through the Third Commandment only to stop dead in their tracks, wondering, "But what about swearing oaths!?""

"Believe it or not, oath-swearing was a hot topic during the reformation. For starters, the Reformers had to think through their pastoral counsel to ex-Catholics who had made monastic vows, often including the promise of lifelong celibacy, and now wanted to break those vows. Calvin considered these vows, since they were 'rash' (i.e., based on future contigencies) and made with intention of meriting God's favor, to be abominable in God's sight. Hence, they not only could be broken, those who made them were duty bound to repent of making them in the first place." 

"More important to the Catechism were the vows sworn to saints or angels. Since God alone can know the heart and God is the one who will hold us accountable for matters of the heart, swearing by any other besides Him is an affront to His authority." Kevin DeYoung. The Good News We Almost Forgot. 175.

Question 101: But may we swear an oath in God's name if we do it reverently?

Answer: Yes, when the government demands it, or when necessity requires it, in order to maintain and promote truth and trustworthiness for God's glory and our neighbor's good. Such oaths are approved in God's Word and were rightly used by Old and New Testament believers.

Question 102: May we swear by saints or other creatures?

Answer: No. A legitimate oath means calling upon God as the one who knows my heart to witness to my truthfulness and to punish me if I swear falsely. No creature is worthy of such honor.