Heidelberg Catechism, Lord’s Day 37
At the same time, as Heidelberg points out, not only Old Testament prophets but also New Testament apostles use oaths (Rom 1:9; 9:1), and so does God himself (Heb 6:16), so it must be possible to take them reverently. Reflecting on what the difference may be, the Catechism adds an explanatory Lord's Day of discussion, before turning to the fourth commandment next week.]
Q101. But may we swear an oath in God’s name
if we do it reverently?
A101. 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 grounded in God’s Word
and were rightly used by the people of God
in the Old and New Testaments.
Q102. May we also swear by saints or other creatures?
A legitimate oath means calling upon God
as the only one who knows my heart
to witness to my truthfulness
and to punish me if I swear falsely.
No creature is worthy of such honour.