Many therefore of his disciples — i.e., of the disciples in the wider sense; those who more or less fully were accepting His teaching, and were regarded as His followers. From John 6:64, the Apostles would seem to be included in the more general designation. In John 6:67 they are separately addressed.
This is an hard saying; who can hear it? — i.e., not hard to be understood, but hard to hear, a stumbling-block in the way of their faith. For the word itself, comp. Matthew 25:24. His meaning was, indeed, not read by them, but the literal meaning was painfully clear, and one to which they will not listen. (Comp. John 10:20.) They do not raise any formal objection to Him, but friends and companions who had talked together of the Teacher and His teaching before, talk again now, and many of them who have followed Him up to this point can follow Him no more.