John 6:60 (NIV)
This is a hard teaching. Who can accept it?
It's always hard for a man to overcome mental barriers to enter the kingdom of God. In the initial stages, he may have to overcome his prejudice that there is no divine creator or a sovereign God. Should he come to Christ by the grace of God, he may find it hard to shed his old life and follow Christ's way of holiness.
In his mid-Christian life, he may find it hard to unlearn some of the theology he accepted to be true. He may have been taught that Christ is divine but doubts that the Lord violated His natural laws like gravity when He walked on water or ascended into heaven. It'll then be hard for him to believe that Christ performed those miracles.
In the last stages of his life, he may find it hard to defeat the skepticism and doubt that begins to creep into his mind.
In the above verse, many of the disciples following Christ hit a roadblock when Christ commanded they had to eat of His flesh and drink of His blood to remain in Him (John 6:56). They found it hard to accept such barbaric and fake teaching.
A saint will face many such mental or spiritual blocks when they have to decide to either accept the words of Christ literally or write them off as just another allegory. Those that swallow those hard teachings go on to glory, while those who turn away because of the vulgarity of it fall out.