Isa 51:2

“For the divine voice says somewhere in the prophecy of Isaiah, "Consider Abraham, your father, and Sarah, who gave you birth." Scripture gives this admonition to those who wander outside virtue. Just as at sea those who are carried away from the direction of the harbor bring themselves back on a course by a clear sign, on seeing either a beacon light raised up high or some mountain peak coming into view, in the same way Scripture by the example of Abraham and Sarah may guide again to the harbor of the divine will those adrift on the sea of life with a pilotless mind.”

“He is leaving aside the unbelievers here in order to address those who have believed: it is to them that he gives the name of enamored with God and with righteousness. He comforts them because they are few in number and invites them to turn their eyes toward their ancestors: he reminds them of Abraham and Sarah and the many thousands of descendants who came from them. This is similar to the term expressed in the divine Gospels: "Do not fear, little flock, for it is your Father's good pleasure to give you the kingdom." It is the same here.”

“Nothing made is an obstacle to my power: not the fact that I called just one man or that old age was on him, or that Sarah lacked the physical capability; yet his race was [indeed] increased, just as I had wished. Be unbelieving no longer, therefore, even though you are [now] easy to count; I will make you too many to number.”

“who bore you Heb. תְּחוֹלֶלְכֶם [lit. shall bear you.] for when he was but one I called him For he was one single person in the land of Canaan where I exiled him from his land and from his birthplace. I called him, meaning that I raised him and exalted him. An expression [similar to] (Num. 1:16) “Those called of (קְרִיאֵי) the congregation.” And just as he was a single person and I exalted him, so will I exalt you, who are singled out to Me.”

“תחוללכם That bare you. According to R. Jonah, it is future, like ,תשובבכם instead of the past חוללתכם. R. Moses Hakkohen believes it to be an adjective, although there is no other adjective of the same form. וַאֲבׇרְכֵהוּ And blessed him. The ו has Pathah, because ואברכהו is future: it means, I told him that I should bless him; and so it was. In the same way.”

“923. Concerning the second, he does three things. First, he gives an example: look unto the rock, namely, Abraham, because of his old age, whence you are hewn, as though violently, not according to the course of nature; and to the hole of the pit, namely, Sarah, to whom it had ceased to be after the manner of women (Gen 18:11): neither did he consider his own body, when it was already dead (Rom 4:19), in which is shown that God has power to make fertile; for I called him alone (cf. Gen 12:1): Abraham was one (Ezek 33:24).”

“alone--translate, "I called him when he was but one" (Eze 33:24). The argument is: the same God who had so blessed "one" individual, as to become a mighty nation (Gen 12:1; Gen 22:7), can also increase and bless the small remnant of Israel, both that left in the Babylonish captivity, and that left in the present and latter days (Zac 14:2); "the residue" (Isa 13:8-9).”