"God had to bring Abraham to the end of his own strength and to let him see that in his own body he could do nothing. He had to consider his own body as good as dead, and then take God, for the whole work; and when he looked away from himself, and trusted God alone, then he became fully persuaded that what He had promised, He was able to perform. That is what God is teaching us, and He has to keep away encouraging results until we learn to trust without them, and then He loves to make His Word real in fact as well as faith."
A. B. Simpson, quoted in Streams in the Desert (emphasis mine)