Men generally swear by him who is infinitely greater than themselves, and an oath for confirmation, to confirm what is promised or asserted, usually puts an end to all contradiction. This shows that an oath taken in a religious manner is lawful even under the gospel: otherwise the apostle would never have mentioned it with so much honour, as a proper means to confirm the truth
Related Commentaries of Hebrews 6:16
Hebrews 6:16
16 For men verily swear by the greater: and an oath for confirmation is to them an end of all strife.