What does John 10:26 mean?

Jesus' religious critics have disputed His teaching and miracles, going as far as to attribute His works to Satan (Mark 3:22). As Jesus has stated, their stubbornness is exactly that: a willful refusal to follow truth (John 5:39–40). They cannot understand because they don't want to understand (John 7:17). Now, these same religious leaders—collectively referred to in John's gospel as "the Jews"—are daring Jesus to repeat His claims. They've surrounded Him in a part of the temple which offers no escape route (John 10:24), seem already prepared for violence (John 10:31), and have challenged Him to say something everyone knows they don't want to hear.

Here, Jesus reiterates a point he made for these men a few months prior: they don't hear His voice because they are not His "sheep" (John 10:1–6). Like sheep, which only recognize the voice of their specific master, these men are practically deaf to the voice of God. As pointed out before, these men are ultimately proving that their master is the Devil (John 8:42–47). Jesus' voice is God's voice (John 10:30); if they don't hear the voice of God, it means they're not part of His "flock."

All the evidence and reason in the world won't make the slightest difference to someone committed to disbelief.