Charles Spurgeon Commentary Romans 4:19-21

"And without being weakened in faith he considered his own body now as good as dead (he being about a hundred years old), and the deadness of Sarah`s womb; yet, looking unto the promise of God, he wavered not through unbelief, but waxed strong through faith, giving glory to God, and being fully assured that what he had promised, he was able also to perform." — Romans 4:19-21 (ASV)

And being not weak in faith, he considered not his own body now dead, when he was about an hundred years old, neither yet the deadness of Sarah's womb: he staggered not at the promise of God through unbelief, but was strong in faith, giving glory to God; and being fully persuaded that, what he had promised, he was able also to perform.

That is the kind of faith we want, the faith that does not enquire how God can perform his promise, but believes that he will do it.