He did not waver at the promise of God through unbelief, but was strengthened in faith, giving glory to God, and being fully convinced that what He had promised He was also able to perform. (Rom_4:20-21)
Faith is the proper response to the promises of God. Also, our faith in God's promises can increase day by day, year by year. These present words offer helpful insight into being strengthened in faith regarding God's promises.
It is so often the case that the circumstances we are in tend to cast doubt upon the promises God has made. This was certainly what happened to Abraham concerning the promised son, Isaac. It was obvious in God's first set of promises to Abraham that a son would be provided some day. "I will make you a great nation" (Gen_12:2). Soon thereafter, the Lord promised that the land He had for Abraham would go to his seed. "Then the LORD appeared to Abram and said, 'To your descendants I will give this land' " (Gen_12:7). The years rolled by, and no son had arrived. Eventually, a son was specifically included in God's promises. "And behold, the word of the LORD came to him, saying, 'This one (Eliezer, his servant) shall not be your heir, but one who will come from your own body shall be your heir' " (Gen_15:4). Still, the years passed by without the arrival of the son.
Now, as Abraham approaches one hundred years of age, God repeats promises that necessitate a son. "When Abram was ninety-nine years old, the LORD appeared to Abram and said to him, 'I am Almighty God . . . I will make My covenant between Me and you, and will multiply you exceedingly... And I will establish My covenant between Me and you and your descendants after you' " (Gen_17:1-2, Gen_17:7). More time passes. Once again, the Lord restates His promise of a son. "Sarah your wife shall have a son" (Gen_18:10). At this point, the scriptures record the natural impossibility of this promise being fulfilled. "Now Abraham and Sarah were old, well-advanced in age; and Sarah had passed the age of childbearing" (Gen_18:11).
Nevertheless, "He did not waver at the promise of God through unbelief, but was strengthened in faith." How did he grow in faith? His circumstances gave reason to doubt: "his own body, already dead (since he was about a hundred years old), and the deadness of Sarah's womb" (Rom_4:19). He focused upon the ability of the God who had promised a son, and he was assured: "being fully convinced that what He had promised He was also able to perform."