Faith is guaranteed by the nature of God Himself. God is absolute truth. He cannot change and He cannot lie-this is what “immutable” means. In order to make His promises and His gospel secure, He backed them up by covenant oath. He swore this oath by none other than Himself, the greatest of all. He has put His own character on the line. 

“For men verily swear by the greater: and an oath for confirmation is to them an end of all strife. Wherein God, willing more abundantly to shew unto the heirs of promise the immutability of his counsel, confirmed it by an oath: That by two im- mutable things, in which it was impossible for God to lie, we might have a strong consolation, who have fled for refuge to lay hold upon the hope set before us “(Heb. 6:16-18).