What does John 8:15 mean?

Contrary to popular belief, Jesus never gave a blanket command that Christians ought not to judge at all. Rather, all Jesus' remarks about judgment were in the context of judging appropriately (John 7:24). Here, Jesus points out that the Pharisees are using flawed, unspiritual judgment. On the other hand, Jesus is not yet executing judgment, though He has the right and the authority to do so! At this point in His ministry, Jesus' role is not to bring judgment for sin (John 3:17)—that judgment will come later, on those who reject Him and His message (John 3:18, 36).

Jesus is responding to Pharisees who challenged His claim to be "the light of the world" (John 8:12). While issues such as miracles and Scripture are subject to human testimony (John 5:36–41), Jesus is now speaking of heavenly matters. On those, only He has true knowledge. Only He can speak truthfully about them. Even so, Jesus can point to two separate "persons" to support His claim: Himself and God the Father (John 5:18). The Pharisees' earlier claim that Jesus' "I am" statement cannot be accepted, then, is missing the mark. Jesus' unflattering remarks about these religious leaders are part of an escalating pattern in this dialogue, which will end with the Pharisees in a murderous rage.