By faith Noah, when warned about things not yet seen, in holy fear built an ark to save his family. By his faith he condemned the world and became heir of the righteousness that comes by faith.
Noah reacted to something that God told Him. Not only did he respond in faith simply because God had told him to do something, he responded with faith that this was even possible. Until the time of Noah, it had never rained on the earth so Noah did not even have a concept as to what was going to happen, only that God said it would.
Faith has a condemning value to it however. As Noah set to building the ark, undoubtedly he attracted attention. When people asked what he was doing and why, he was certainly mocked and ridiculed. They were given the same information that Noah had been. God would flood the earth. But they did not react in faith in the same way that Noah had and thus were swept up in the flood when it came.
People with more evidence are held more accountable for having faith. As Jesus pronounces woe on the cities of Israel He notes that if the same miracles performed in them had been performed in Sodom and Gomorrah, Tyre and Sidon, they would have believed and repented.
The people who saw Noah building the ark had the same evidence of a coming flood as Noah did – only the word of God. Noah responded in faith and was saved while the others were condemned as the door to the ark was shut.