Hebrews 6:16-19

For men indeed swear by a greater, and with them the oath is a term to all dispute, as making matters sure. Wherein God, willing to shew more abundantly to the heirs of the promise the unchangeableness of his purpose, intervened by an oath, that by two unchangeable things, in which it was impossible that God should lie, we might have a strong encouragement, who have fled for refuge to lay hold on the hope set before us, which we have as anchor of the soul, both secure and firm, and entering into that within the veil