What does John 10:21 mean?

In Jesus' era, insanity was assumed to be caused by demonic possession. Whether or not it always was, such was the perception of His culture. When Jesus' critics accuse Him of "having a demon" (John 8:48–52), they're saying He's crazy. Jesus' public actions have led to occasional claims that He's mentally ill, including from His own family (Mark 3:21). In this case, Jesus recently claimed He will voluntarily die, and just as voluntarily rise from the dead, thanks to special authority granted Him by God (John 10:17–18). As expected, that caused a sharp controversy among those listening in (John 10:19).

Some, like prior critics, hear those extravagant claims and write Jesus off as unstable. That response is rooted partly in spiritual stubbornness: such people are not willing to consider God under any circumstances (John 10:1–6; 7:17; 5:39–40). They aren't open to seeing Jesus' miracles as proof that He is everything He claims to be (Matthew 12:31; Luke 11:15).

In contrast, those miracles are the main point referenced by the other major faction in the crowd. Like others before, they rightly recognize these are "signs" meant to give divine approval to Jesus' message (John 3:1–2).

After this, John's gospel skips forward several months to the Feast of Dedication (John 10:22). We don't learn, at least, here, how exactly the different factions in the crowd continue their debate over Jesus and His healing of the formerly blind man.