The Heidelberg Catechism: Lord's Day 37
101. Question: But may we swear an oath by the Name of God in a godly
Answer: Yes, when the government demands it of its subjects, or when necessity requires it, in order to maintain and promote fidelity and truth, to God's glory and for our neighbor’s good. Such oath-taking is based on God's
Word and was therefore rightly used by saints in the Old and the New Testament.
 Deut. 6:13; 10:20; Jer. 4:1, 2; Heb. 6:16.  Gen. 21:24; 31:53; Josh. 9:15; I Sam. 24:22; I Kings 1:29, 30; Rom. 1:9; II Cor. 1:23.
102. Question: May we also swear by saints or other creatures?
Answer: No. A lawful oath is a calling upon God, who alone knows the heart, to bear witness to the truth, and to punish me if I swear falsely. No creature is worthy of such honour.
 Rom. 9:1; II Cor. 1:23.  Matt. 5:34-37; 23:16-22; James 5:12.
The Heidelberg Catechism
Lord's Day 1