Isaiah 50:7

For the Lord God will help Me;

Therefore I will not be disgraced;

Therefore I have set My face like a flint,

And I know that I will not be ashamed.

Believing is a decision. The best time to believe that God can do something for us is before we get into a tight position.  What we believe at that moment will either bring us to victory or defeat.  What is flint?  A stone that was used as cutting tool for a knife before they had metal.  When we set our face like flint, we believe and cut out any doubt that would try to persuade us otherwise.