"He did not weaken in faith when he considered his own body, which was as good as dead (since he was about a hundred years old), or when he considered the barrenness of Sarah's womb.
"No distrust made him waver concerning the promise of God, but he grew strong in his faith as he gave glory to God, fully convinced that God was able to do what he had promised."
Abraham chose to trust in God's Word more than he trusted the evidence before his eyes; he chose to lean not on his own understanding, but to trust in God with all of his heart. Abraham made this choice because the unseen God was more real to him than the things which are seen.
It is also interesting to note that Abraham's faith grew strong "as he gave glory to God." Abraham gave glory to God-- praised Him for His goodness, thanked Him for His blessings, and attributed his success to God-- all while waiting in earnest hope for God's promise to be realized. This was what strengthened Abraham's faith, his trust in God-- thanking and praising God for the gifts of the present while waiting for the promise of the future.
May God grant me the strength to praise Him for His goodness and thank Him for His blessings today that I may strengthen my trust to believe in His promises for my future.