When many of his disciples heard [what Jesus had to say], they said, “This teaching is difficult; who can accept it?” 
Anyone who has heard the teaching of Jesus has to sympathise with those first disciples. His words about Holy Communion may no longer have the same power to shock seasoned churchgoers, because it's an established ritual of the Church which many of us share in without any more thought than we would give to a picnic in the park. But, for many people outside the Church, the idea that we can meet Jesus simply by sharing bread and wine is at best ludicrous and at worst a serious stumbling block to faith.
I guess it offends them in the sense that it offends against their notion of common sense. Perhaps they would find it easier to accept if they understood that Jesus is not proposing any magical or supernatural change to the bread and wine we share. He is simply promising to be with us, in spirit, as we come together around the communion table.
The sharing of holy communion is simply a way of cashing in that promise. It gives us the special confidence of knowing that Jesus is indeed with us, even when he feels far away, because he promised to be there when we share the bread and wine together.
However, let us leave on one side the meaning of holy communion. What about the rest of Jesus' teaching, especially his teaching about lifestyle and morality? This teaching is difficult. Who can accept it?
No wonder that some people do not believe in trying to put God first, and to love our neighbours as much as we love ourselves. No wonder either that there are some people who claim to believe in these values, but who betray them when they are put to the test. No wonder people turn back and no longer go with Jesus and his followers. It ain't easy to be a true Christian!
So why do any of us try to persevere? Because as St Peter put it, even if we wanted to follow someone else's way, 'Lord, to whom could we go? You have the words of eternal life. We have come to believe and know that you are the Holy One of God.'
Peter himself was to betray Jesus when it came to the crunch, but he didn't allow that to stop him from coming back to the teaching which he knew, from bitter experience, is difficult to follow, but which he also knew to be life enhancing and life giving.
 John 6.60—69