the English word "Believed" there in the Greek contextually is speaking of Abraham trust in God as able to aid either in obtaining or in doing something not that he was accepting there was a God Abraham's understanding of God was far more than agreeing there is one. Abraham Knew God.
Believed = to have faith (in, upon, or with respect to, a person or thing), i.e. credit; by implication, to entrust (especially one's spiritual well-being to Christ):—believe(-r), commit (to trust), put in trust with. requires action.