What does John 5:44 mean?
This comment matches the theme of verse 43. Those religious leaders who reject Jesus do so in a hypocritical way. They place great emphasis on traditions (Matthew 15:6–9), which are only the ideas of other men. They place themselves on pedestals above others (John 9:34). They claim to have great knowledge (John 9:28–29). And yet, they reject evidence related to Jesus because they refuse to believe in Him (John 5:39–40).This refusal shows the truth of their so-called-faith in God. This is a consistent teaching in the New Testament: what a person does proves what they really believe (James 2:14). In verse 42, Jesus said that He "knew" these men did not really love God, because they are rejecting His true messenger (John 5:38). Jesus has provided plenty of evidence proving His claims (John 5:30–40), but that will never be enough for those who have made up their minds to reject God.
This is the "belief" Jesus is referring to here: an actual trusting faith in God. This is impossible for those willing to take the word of other men over the testimony of God.