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 more influential than hearing 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 (John 5:38), so they don't recognize the One God sent as His living message (John 1:18).

Scripture 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. This applies to any concept, but especially to personal issues such as faith. 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 different from "trusting" (James 2:19), and so mere "knowledge" is different from 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.