Do not use an oath. Jesus said this same thing in Matthew 5:33-37 and notes there. Our modern sin of profanity grew out of the habit of using an oath (vow) to affirm a promise. Since Jews believed some oaths were binding, but others were not, MacKnight thinks some were falsely swearing an oath and denying they were Christians to avoid persecution. They would do this with their "fingers crossed," expecting God not to hold them to it.

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Old Testament