What does John 15:25 mean?

Sin is inherently irrational. Despite significant efforts to justify rejection of God, nothing in human experience makes sense outside of the Christian worldview. Every other approach ends in blatant contradiction, or despair, or both. Those who hate and revile the gospel aren't acting reasonably. They're reacting, like instinctive animals (2 Peter 2:12), according to the temptations of Satan (John 3:19; 8:44). The more a person knows—or ought to know—about God, the more obvious their rejection of Him will be (John 5:39–40; 9:39–41).

Jesus is probably quoting Psalm 109:3, though Psalm 35:19 and Psalm 69:4 use similar phrases. Belief in God is not "only" moral, spiritual, or emotional. It is also rational: it is the expected response of beings created for a purpose and given all the tools necessary to realize it (Romans 1:18–20; Psalm 19:1; Matthew 7:7–8). To reject God is not "only" immoral, unspiritual, or emotional. It is also irrational: it represents beings rebelling against their created purpose and ignoring what they already know (1 Peter 4:4).