Charles Ellicott Commentary Matthew 26:72

"And again he denied with an oath, I know not the man." — Matthew 26:72 (ASV)

With an oath —Once the downward step was taken, the disciple's fall was fatally rapid. Forgetful of his Lord's command forbidding any use of oaths in common speech (Matthew 5:34), he did not shrink from invoking the divine name, directly or indirectly, to attest his falsehood.