I've heard it taught that Noah warned the world about the flood, but I can't find that in the Bible. On the contrary, Jesus says in Matthew 24 that the flood was a surprise to the world. Why do people believe Noah warned the earth?
The Biblical record provides only a glimpse of Noah's activities in the days leading up the flood, and most of the narrative in Genesis is focused on his work in building the ark. Nevetheless, we can make a reasonable assumption that Noah preached about the coming flood to his neighbors and community during the 100 years required to complete the ark.
First, we know from Genesis 6 that Noah was a blameless man, which means he lived a godly lifestyle before others. Furthermore, 2 Peter 2 tells us Noah was a preacher of righteousness:
Knowing that Noah was a preacher proclaiming God's truth in some form to the people of his day and given his godly character, we can assume Noah preached about the approaching flood and the need to repent. Furthermore, offering the world such a warning would be in keeping with God's character, since God typically gives opportunity for repentance prior to His judgments.
If Noah did warn the world, the world obviously chose to ignore his warnings, because the Bible teaches that the world was taken by surprise when the flood came. The fact that the world was caught off guard by the flood does not prove that Noah never preached concerning the coming judgment. It only tells us that the world paid no attention to the warnings.
In fact, men have heard warnings from many prophets over the ages, yet they usually ignore the warnings and are caught off guard by the judgment that follows. The entire history of the nation of Israel testifies to this truth.
Furthermore, Matthew 24 goes on to record Jesus Himself warning our world of another coming judgment, His Second Coming. Nevertheless, Jesus says the world will be caught off guard when He returns:
Notice, the world will be surprised by the Second Coming despite the Bible's many warnings (often repeated by preachers throughout the centuries) that this event will lead to judgment for the ungodly. If Jesus' warnings will be ignored before His Second Coming, then certainly Noah's warnings could have been ignored by his world before the flood.