Albert Barnes Commentary Isaiah 51:2

"Look unto Abraham your father, and unto Sarah that bare you; for when he was but one I called him, and I blessed him, and made him many." — Isaiah 51:2 (ASV)

Look unto Abraham - What was figuratively expressed in the former verse is here expressed literally. They were directed to remember that God had taken Abraham and Sarah from a distant land, and that from so humble a beginning He had increased them to a great nation. The argument is, that He was able to bless and increase the exiled Jews, though comparatively feeble and few.

For I called him alone - Hebrew, For one I called him; that is, he was alone; there was but one, and he increased to a mighty nation. So Jerome, Quia unum vocavi eum. So the Septuagint, Ὅτι εἷς ἦν hoti heis ēn — For he was one. The point of the declaration here is, that God had called one individual—Abraham—and that He had caused him to increase until a mighty nation had sprung from him, and that He had the same power to increase the little remnant that remained in Babylon until they should again become a mighty people.