What does John 8:18 mean?

When speaking on earthly issues, Jesus accepted the need to provide evidence (John 5:30–47). In that context, He pointed to His own miracles, the evidence of Scripture, and the testimony of other people to prove His claims. Here, however, Jesus is defending an inherently spiritual claim. Earlier, Jesus said He was "the light of the world" (John 8:12). This is knowledge which no other person could have—so for Jesus to testify about Himself is legitimate.

Further, those who oppose Him are using improper judgment in order to dismiss Him. Not only is it valid for Jesus to testify about something He alone has seen, He also has the support of an additional witness: God the Father. In this case, Jesus is once again pointing to the miracles He has performed as evidence that God supports His ministry (John 20:30–31). This is a key part of the gospel of John's proof that Jesus is not only Messiah, but God incarnate. Even by the Pharisees own law and logic, miracles as proof of God's approval should be an acceptable argument (John 3:1–2).

Of course, logic and law are not the primary goal of the Pharisees in this moment. Rather than wrangle this point, they will attempt to shift the conversation, challenging Jesus' references to God as His father.