Q. 101. May we then swear religiously by the name of God?
Yes: either when the magistrates demand it of the subjects; or when necessity requires us thereby to confirm fidelity and truth to the glory of God, and the safety of our neighbour: for such an oath is founded on God’s Word, and therefore was justly used by the saints, both in the Old and New Testament.
 Exodus 22:11; Nehemiah 13:25;  Deuteronomy 6:13; Hebrews 6:16;  Genesis 21:24; Joshua 9:15, 19; 1 Samuel 24:22; 2 Corinthians 1:23; Romans 1:9.
Q. 102. May we also swear by saints or any other creatures?
No; for a lawful oath is calling upon God, as the only one who knows the heart, that He will bear witness to the truth, and punish me if I swear falsely; which honour is due to no creature.
 2 Corinthians 1:23; &nbnsp; Matthew 5:34–35.
During the days of the Reformation, there was a group of Protestants known as the Anabaptists, or the radical Reformers, who believed that it is always wrong to swear religiously in the name of the Lord. Their basis for their conviction is the words of the Lord in Matthew 5:34–37—
Again, ye have heard that it hath been said by them of old time, Thou shalt not forswear thyself, but shalt perform unto the Lord thine oaths: But I say unto you, Swear not at all; neither by heaven; for it is God’s throne: Nor by the earth; for it is his footstool: neither by Jerusalem; for it is the city of the great King. Neither shalt thou swear by thy head, because thou canst not make one hair white or black. But let your communication be, Yea, yea; Nay, nay: for whatsoever is more than these cometh of evil.
At first reading, the words of the Lord do seem to be giving a blanket prohibition of swearing or of oaths and vows. But this cannot be the case, firstly, because the Apostle Paul practised it without giving a hint that he thought it improper (e.g., Rom 1:9); secondly, the writer of Hebrews speaks of God swearing (Heb 6:16–17); and, thirdly, notice that the Lord does not condemn swearing in the Lord’s name, but swearing by the furnishing of the temple. We therefore believe, with the magisterial Reformers, that the Lord was in fact condemning the Jews’ habit of swearing rashly and trivially. Perhaps the Jews had trivialised the making of an oath so much that they knew better than to swear in God’s name!
But to swear by any creatures, whether they be saints or angels, or by any objects, whether sacred or otherwise, would always be sinful. This is because God alone is able to see our hearts, whether we be speaking the truth, and God alone has the power to execute judgement in the case when we break our vows or tell a falsehood. Therefore when a believer swears in the name of God, it is really an appeal to His omniscience, omnipotence and justice. To swear by any other thing is always evil,—not only because it would be a violation of Deuteronomy 6:13, “Thou shalt… swear by his name,” but because it would rob God of the honour due Him and would involve a superstitious idolatry in which the thing swore by is ascribed some power it does not have.
Naturally, since all vows and oaths involve our calling upon God to exercise His holy justice, we must never swear unadvisedly and over minor matters. Oaths and vows ought rather to be taken only in “matters of weight and moment.” This would include testifying in a trial or making important declarations or even in swearing allegiance to a nation—which, of course, would be on condition that it does not compromise our faith and fidelity to the Lord.
Heidelberg Catechism >