God's promises are sure because God cannot lie
Hebrews 6:13–20 (ASV)
13For when God made promise to Abraham, since he could swear by none greater, he sware by himself, 14saying, Surely blessing I will bless thee, and multiplying I will multiply thee. 15And thus, having patiently endured, he obtained the promise. 16For men swear by the greater: and in every dispute of theirs the oath is final for confirmation. 17Wherein God, being minded to show more abundantly unto the heirs of the promise the immutability of his counsel, interposed with an oath; 18that by two immutable things, in which it is impossible for God to lie, we may have a strong encouragement, who have fled for refuge to lay hold of the hope set before us: 19which we have as an anchor of the soul, a hope both sure and stedfast and ente...