John Calvin 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. In order that she might have more courage to bring up her son, God confirms to her what he had often promised to Abraham previously. Indeed, nature itself prescribes to mothers what they owe to their children; but, as I have lately hinted, all Hagar's natural feelings would have been destroyed unless God had revived her, by inspiring new confidence, to apply herself with fresh vigor to fulfilling her maternal office.

With respect to the fountain or “well,” some think it suddenly sprang up. But since Moses says that the eyes of Hagar were opened, and not that the earth was opened or dug up, I rather incline to the opinion that, having been previously astonished with grief, she did not discern what was plainly before her eyes; but now, finally, after God has restored her vision, she begins to see it.

And it is worthy of special notice that when God leaves us destitute of his superintendence and takes away his grace from us, we are as much deprived of all the aids which are close at hand as if they were removed to the greatest distance. Therefore, we must ask, not only that he would bestow upon us such things as will be useful to us, but also that he will impart prudence to enable us to use them; otherwise, we are destined to faint, with closed eyes, in the midst of fountains.