Abraham, the Father of Faith, had a moment of doubt with God in Genesis 15. He asked God how he would know what God said would be true. God did not have to prove anything. His word is good enough. But God still made a covenant with Abraham to prove to Abraham that God meant what He said. God has made a covenent with us as well. What Jesus accomplished on the cross, and the spilling of His blood, was a new covenant that we are now a part of.