Slander & gossip

 To spread lies about another person's character, whether you know it is a lie or not, is one of the most severe sins a person can commit.

That is exactly what Satan has done to God, and look at what he thereby brought upon himself, and consider the level of suffering he has caused God as well as that which those who have believed his lies allowed him to bring upon them. (see John 8:44, Psalm 15 and Proverbs 6:16-19 NKJV)