Adam Clarke on Matthew 23:20

Whoso - shall swear by the altar - As an oath always supposes a person who witnesses it, and will punish perjury; therefore, whether they swore by the temple or the gold, (Mat 23:16), or by the altar or the gift laid on it, (Mat 23:18), the oath necessarily supposes the God of the temple, of the altar, and of the gifts, who witnessed the whole, and would, even in their exempt cases, punish the perjury.