Romans 5:20-21

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 to perform.

When you know you have a promise and believe in something you will not waver. Think about what it is that makes you fully persuaded. We go from unbelief to belief pretty quickly depending on what we hear and who we hear it from. If we become fully persuaded that the word of God is true and final and settled in our hearts we would not stagger in unbelief either.