I even I am he that comforteth you - The word ‘I’ is repeated here to give emphasis to the passage, and to impress deeply upon them the fact that their consolation came alone from God. The argument is, that since God was their protector and friend, they had no occasion to fear anything that man could do.
Of a man that shall die - God your comforter will endure forever. But all men - even the most mighty - must soon die. And if God is our protector, what occasion can we have to fear what a mere mortal can do to us?
And of the son of man - This phrase is common in the Hebrew Scriptures, and means the same as man.
Shall be made as grass - They shall perish as grass does that is cut down at mid-day (see the notes at Isaiah 40:6-7).