Romans 4:18-21
Who against hope believed in hope, that he might become the father of many nations; according to that which was spoken, So shall thy seed be.  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.

We have a strong faith by having a sure faith, and it has very little to do with us.  There were all kinds of things working against Abraham in his situation.  He was old.  His wife was old and past childbearing years.  Yet God had promised a son.  And Abraham didn’t consider himself—his lack and his inability.  Instead, he considered God, a God he knew to be big, strong, powerful, faithful, and able to do all things.  We can have a strong faith, too, by considering not ourselves or our circumstances, but the God who made the promise.

Abraham was not staggered by the promise God had made to him.  He wasn’t thrown off by it or driven to unbelief at the magnitude of it.  He didn’t teeter or waver, but simply continued to trust God.  He was so sure of God, fully persuaded, convinced beyond any doubt that God was believable, God was able, and God would be faithful to do all He said and to keep His word.  We can have that same kind of faith in God for the promises that He gives to us.

So what do we need to believe God for?  In this chapter, Paul is talking about the certainty of our justification through faith in Christ.  Jesus Himself was a sure promise that was given and fulfilled by God.  He came to be all that God said He would be and to do all that God said He would do.  He fulfilled everything necessary to secure our salvation, and so we can rest in that and have that sure knowledge that it is all secured through our faithful and unfailing God.

God gives different promises to different people for different reasons, but it all works generally the same way.  When God says something, we can trust Him fully.  We can believe all His promises and expect Him to be faithful.  There’s no need for doubt or weakness or wavering, just confidence in God and who we know Him to be.  We can know with certainty that God will come through, God will not lie, God will not fail, and there is tremendous power in the kind of faith that is simply willing to take God at His word.