What does John 5:38 mean?
Jesus has been explaining three forms of evidence to support His claims, in accordance with Jewish law (Numbers 35:30; Deuteronomy 17:6). The first two of these were human testimony from John the Baptist (John 5:33) and direct observations from His own miracles (John 5:36). Seeing miracles is a much more powerful evidence than human testimony. However, to the Jewish religious leaders, nothing should have been as effective as the evidence of God's written Word (John 5:37). And yet, these men are rejecting Jesus. According to the prior verse, they have not heard the voice of God, so they don't recognize the One God sent as His living message (John 1:18).This verse explains part of how that is possible. It's a simple thought that human beings often try to ignore: those who refuse to believe will not believe, evidence or not. Jesus will point this out in a later discussion: a person cannot understand the gospel unless they are first obedient enough to hear from God (John 7:17). James, one of the earliest-written books of the New Testament, makes it clear that "knowing" is not the same as "trusting" (James 2:19), and so mere "knowledge" is not the same as saving faith (James 2:14).
This, as Jesus will prove, is the core problem facing people such as the Pharisees. They are more interested in their own traditions (Matthew 15:6), power (John 11:48), and prestige (John 9:34) than the truth. So, even though they've been given all the evidence they need (John 9:41), they do not—they will not—believe (John 5:40). As upcoming verses will note, this is not a matter of knowledge, it is a question of will.