It is well too that Hagar was found “by a spring of water,” for beginners find themselves engaged in purifications, which are signified by water. By contrast, those who are more fully matured come into a desert place, no longer needing purifications, having already rid themselves of vices and having been endowed with virtue.
(Verse 7) And the angel of the Lord found her by a fountain of water in the desert, by the fountain on the way to Shur. Then the Egyptian woman hurriedly went on the way to Shur, which leads through the desert to Egypt.
There was indeed on earth, so long as it was needed, a symbol and foreshadowing image of this city, which served the purpose of reminding men that such a city was to be rather than of making it present; and this image was itself called the holy city, as a symbol of the future city, though not itself the reality. Of this city which served as an image, and of that free city it typified, Paul writes to the Galatians in these terms: "Tell me, you that desire to be under the law, do you not hear the law? For it is written, that Abraham had two sons, the one by a bond maid, the other by a free woman. But he who was of the bond woman was born after the flesh, but he of the free woman was by promise. Which things are an allegory: for these are the two covenants; the one from the mount Sinai, which genders to bondage, which is Agar. For this Agar is mount Sinai in Arabia, and answers to Jerusalem which now is, and is in bondage with her children. But Jerusalem which is above is free, which is the mother of us all. For it is written, Rejoice, you barren that bear not; break forth and cry, you that travail not, for the desolate has many more children than she which has an husband. Now we, brethren, as Isaac was, are the children of promise. But as then he that was born after the flesh persecuted him that was born after the Spirit, even so it is now. Nevertheless, what says the Scripture? Cast out the bond woman and her son: for the son of the bond woman shall not be heir with the son of the free woman. And we, brethren, are not children of the bond woman, but of the free, in the liberty wherewith Christ has made us free." [Galatians 4:21-31] This interpretation of the passage, handed down to us with apostolic authority, shows how we ought to understand the Scriptures of the two covenants — the old and the new. One portion of the earthly city became an image of the heavenly city, not having a significance of its own, but signifying another city, and therefore serving, or "being in bondage." For it was founded not for its own sake, but to prefigure another city; and this shadow of a city was also itself foreshadowed by another preceding figure. For Sarah's handmaid Agar, and her son, were an image of this image. And as the shadows were to pass away when the full light came, Sarah, the free woman, who prefigured the free city (which again was also prefigured in another way by that shadow of a city Jerusalem), therefore said, "Cast out the bond woman and her son; for the son of the bond woman shall not be heir with my son Isaac," or, as the apostle says, "with the son of the free woman." In the earthly city, then, we find two things — its own obvious presence, and its symbolic presentation of the heavenly city. Now citizens are begotten to the earthly city by nature vitiated by sin, but to the heavenly city by grace freeing nature from sin; whence the former are called "vessels of wrath," the latter "vessels of mercy." [Romans 9:22-23] And this was typified in the two sons of Abraham — Ishmael, the son of Agar the handmaid, being born according to the flesh, while Isaac was born of the free woman Sarah, according to the promise. Both, indeed, were of Abraham's seed; but the one was begotten by natural law, the other was given by gracious promise. In the one birth, human action is revealed; in the other, a divine kindness comes to light.
And when the angel found her by the spring of water in the wilderness on the road to Shur, he said to her, "Hagar, maidservant of Sarai, where have you come from and where are you going?" Consequently, the Egyptian on the road to Shur, which leads through the desert to Egypt, was hastening. For the desert of Shur stretches up to the Red Sea, which reaches the borders of Egypt.
[AD 398] Didymus the Blind on Genesis 16:7