“But there are some of you who don’t believe.”
Jesus, in John 6:64
There is a type of believing that was impossible for those who were under the Old Testament law, including Jesus’ own disciples. When Jesus came and purchased God’s Spirit for us by his sacrificial death, he brought about a new kind of life, with a new kind of believing. Of course, Jesus’ disciples believed as much as humans could possibly believe without the Spirit, and they loved Jesus as much as humans could love him, but God’s love would fill their hearts when the Spirit came, and then they would be capable of a faith they previously knew nothing about.
But Jesus also had some disciples who did not believe at all. They followed him even to remote places, and they listened to him teach; still, they did not believe in him. Of course, if you had asked them if they believed, they would have said they certainly did, but it was not so. It seems impossible that such a thing could be, but it is actually very common, even today. There have always been people who are attracted by the happiness of faith that they see in others, even though they do not themselves have faith. They want the benefits of righteousness that they see in others, but they do not have a heart for it themselves. In John 6, we are given an example of such disciples of Jesus:
60. When many of his disciples heard this, they said, “This is a hard saying; who can hear it?”
61. But Jesus, knowing in himself that his disciples were grumbling about this, said to them, “Does this offend you?
62. Then what if you should see the Son of man ascending to where he was before?
63. It is the Spirit that gives life; the flesh is worthless. The words that I speak to you, they are spirit, and they are life.
64. But there are some of you who don’t believe.” (For Jesus knew from the beginning who they were who didn’t believe and who it was who would betray him.)
Because these men did not believe, but were following Jesus for their own misguided reasons, they could not endure the doctrine he taught. Jesus knew from the beginning that these men did not believe, but instead of telling them not to follow him, he simply did his work and let them figure it out for themselves. Eventually, on their own, they came to the conclusion that Jesus was not the man they expected him to be. They forsook him, and they no doubt considered themselves fortunate to have escaped from what they would have called the Jesus of Nazareth cult.
It is ironic that what those unbelieving disciples did is recorded in verse 6:66:
John 6:66: From that time, many of his disciples turned back to former things and no longer walked with him.