John Gill Commentary Genesis 21:18

"Arise, lift up the lad, and hold him in thy hand. For I will make him a great nation." — Genesis 21:18 (ASV)

Arise, lift up the lad
She had set herself down at some distance, and now she is bid to rise up and go to the place where she had left her son, and raise him up from the ground, on which he lay along:

and hold him in your hand :
or take hold on him with your hand, and hold him up with it, he being so weak that he could not sit up without being supported:

for I will make him a great nation :
which is a renewal of a promise before made both to her and to Abraham, (Genesis 16:10) (17:20) (21:13) ; and by this Hagar is assured that he would recover and live, and become a man and the father of children, who in time would become a great nation; (See Gill on Genesis 16:10), (See Gill on Genesis 17:20), (See Gill on Genesis 21:13), this shows that the Angel of God here speaking is God himself, or a divine Person, since none but he could make him a great nation.