Faith and trust are related but distinct concepts. In relation to God, faith is the belief that He exists and that His word is true and reliable. Trust, on the other hand, involves acting on His word especially when one cannot see how that will turn out. When Abraham was told to sacrifice his son, he had no doubts about God's existence or that he had heard His word. So he obeyed, trusting that somehow God would honor His promise to establish a covenant with Isaac.