It is not possible to be an uncommitted follower of Jesus Christ. Either you are for him or you are against him (Luke 11:23). There is no middle ground. Jesus has just proclaimed to be a suffering Messiah who would die on behalf of his people. Though the crowd might not have understood everything that Jesus was saying, they understood well enough to get mad and leave.
Jesus is now speaking to his closest followers of all, both the twelve disciples and others who have been faithfully learning from him. He presents them with a decision that they must make: they may either follow him without doubting him at all, or leave him altogether. Most choose to leave.
Jesus could see into the hearts of his hearers, and he knew that they did not like what he had to say. His words had been offensive to many of them, but if they were offended by this, they would certainly be much more offended by his upcoming death and resurrection. To them, it would seem like his ultimate defeat, though we understand that is is his greatest victory.
The difference between those who do not believe and those who do is the work of the Holy Spirit. He is the one that “gives life” (John 6:63) and the one that causes people to come to the Father (John 6:65). He is the one who enables men to lift their eyes up from earthly things to the heavens, so that they are able to understand God and what he requires of them.
Jesus turns to the most faithful of the faithful, the twelve disciples, and he asks them if they will leave as well. The words that Peter gives in response are the true confession of all Christians everywhere, “Lord, to whom shall we go? You have the words of eternal life, and we have believed, and have come to know, that you are the Holy One of God.” (John 6:68-69). He is God Almighty, the giver of life, the redeemer of his people.
Of course, Jesus knew that Peter was exaggerating a bit when he said “we have believed, and have come to know.” Not all of the twelve truly believed and had truly come to know that Jesus is the God who is to be worshiped and adored. A traitor was in their midst.