Chapter 36: Lord's Day 36
Section 101. Q. 101: But may we swear an oath in God's name if we do it reverently? A. 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 Testament saints.
Section 102. Q. 102: May we also swear by saints or by other creatures? A. 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.