What would have happened if Noah had believed a flood was coming but never built the ark?
If Noah had only believed a flood was coming but never built the ark, he would have been washed away along with the rest of the people. His faith had to be accompanied by action, so he built the ark as God had warned.
Les explains from Hebrews 11:7 that Noah was warned by God of things not seen as yet, moved with fear, and built an ark. He did something. If Noah had stood there saying he believed a flood was coming but never built the ark, he would have gotten washed away right along with the rest. Along with his faith that a flood was coming, he built the ark. Les uses this to show that these Old Testament men of faith were not faith alone; their faith was connected with works. By contrast, Abraham simply believed God, and God imputed righteousness to him without his doing a thing. So it is with us: without doing a thing, we simply believe that Christ died for us and was raised in resurrection power.
By faith Noah, being warned of God of things not seen as yet, moved with fear, prepared an ark to the saving of his house; by the which he condemned the world, and became heir of the righteousness which is by faith.