Genesis 15:6 And he believed in the LORD; and he counted it to him for righteousness.
Abraham believed God. When he believed then God counted it to him for righteousness. It wasn’t anything that Abraham did. He simply believed that God would do what He said He would do!
By the way, it wasn’t the power of his faith in God. Actually, his faith was pretty weak. If you keep reading you will realize that he keeps doubting God and trying to solve the problem in his own strength. He hoped that his efforts could help God keep His Word.
So it is not the power of faith but faith in the One that has the power to do what He promises. We believe God. We believe His Word. We trust Him. That is how He is pleased.
We don’t earn it or deserve it! He just counts it to us!