John 6:62 Meaning

60Many therefore of his disciples, when they had heard this, said, This is an hard saying; who can hear it?

61When Jesus knew in himself that his disciples murmured at it, he said unto them, Doth this offend you?

62What and if ye shall see the Son of man ascend up where he was before?

63It is the spirit that quickeneth; the flesh profiteth nothing: the words that I speak unto you, they are spirit, and they are life.

64But there are some of you that believe not. For Jesus knew from the beginning who they were that believed not, and who should betray him.