Many therefore of his disciples, when they had heard this, said, This is an hard saying; who can hear it?
When Jesus knew in himself that his disciples murmured at it, he said unto them, Doth this offend you?
And he said, Therefore said I unto you, that no man can come unto me, except it were given unto him of my Father.
From that time many of his disciples went back, and walked no more with him.

COMMENTARY

This is an hard saying; who can hear it?
From that time many of his disciples went back, and walked no more with him.
We learn from this passage that Jesus lost many of his followers after teaching a controversial principle of the gospel. The offending doctrine was that of the sacrament. He had just taught the need to partake of his flesh and blood, so that his disciples could live through him.
Interestingly, the ancient Jews had been being prepared to receive this doctrine for millennia, as the consumption of sacrificial animals was a core element of temple worship. Perhaps they were too attached to this shadow of the true principle, to then accept the fully embodied version when it came to them.

No man can come unto me, except it were given unto him of my Father.
One day, like these disciples, we will each realize that we have misunderstood one principle of the gospel or another. Which principle you stumble over will probably be different from the one that challenges me. Perhaps I have not correctly perceived grace, or repentance, or tithing. Perhaps you have misunderstood faith, or the Holy Spirit, or sacrifice.
Proper understanding of these, and all other principles, can only come from one source. As Jesus explained, God Himself must enlighten our minds and hearts to receive pure truth.
The question is how we will respond when that greater understanding is presented to us. Everyone thinks that they will accept truth when they find it, but it turns out that many of us are more attached to what we thought was right, than what is actually right.