adjective1. incapable of producing any result; ineffective; useless; not successful: Attempting to force-feed the sick horse was futile.
2. trifling; frivolous; unimportant
Romans 1:21 says, “For although they knew God, they did not honor him as God or give thanks to him, but they became futile in their thinking, and their foolish hearts were darkened. ”
I can know God and still not honor him. I can know God and not give thanks to him. I can know God and still have a foolish heart. I can know God and all my thinking be completely useless.