Hebrews 11:11-12
Through faith also Sara herself received strength to conceive seed, and was delivered of a child when she was past age, because she judged him faithful who had promised.  Therefore sprang there even of one, and him as good as dead, so many as the stars of the sky in multitude, and as the sand which is by the sea shore innumerable.

We can have faith because God is faithful.  His character revealed to us proves that.  Sarah was able to bear a child in her old age because God gave her the strength to do so as she trusted Him.  She understood that the God who told her she would bear a child would keep that promise and that she would actually experience that in her life.  And it was all based on her view of God.  Through that one child, a nation was formed, but it all began with one act of faith.

So what is our view of God?  Have we judged Him faithful?  Do we believe the promises that He has given to us?  We do have a choice in the matter.  We can either take God at His word, or we can give in to doubt.  We can live like we believe Him and conduct our lives accordingly or reap the consequences of living a faithless life.  But God will always be true, so we can have absolute and unquestioning faith in Him.  We can see the impossible become possible.

It’s this kind of faith that understands we really have nothing to contribute.  Sarah couldn’t offer much to this situation.  She was past age, too old to get pregnant.  It was physically impossible.  But God is able to give life in a dead place.  He is able to add something to what she lacked.  In many ways, Sarah simply had to be a willing vessel.  And isn’t that all that God requires of us?  He’ll do the filling if we’ll hold still and trust Him to pour what we lack into our lives so He can use us for His purposes.

So can’t we just let go and trust God wholeheartedly?  He’s given us all the proof we need to judge Him faithful.  He’s never let us down.  He’s never failed.  He’s fulfilled everything He promised.  It’s up to each of us to work through whatever it is that implants doubt in our hearts—whether it’s pride or fear or misunderstanding or anything else that keeps us in our unbelief.  And when we can do that, we can create a legacy of faith that spreads farther and wider than we could imagine.