If you want to get a loan from a bank, you need to get a guarantor, If you are not able to pay back the loan. then the guarantor needs to take responsibility. After God promised and made an oath with us, He wanted to establish a guarantor to show us that He will keep His promise, However, if you want to have a guarantor, you need one that is greater than God, Because there is no such being, God, Himself is the guarantor. If there are problems with the promise, then God said He would take full responsibility.