John Calvin 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)

For I called him alone. This application plainly shows what was the purpose of this exhortation from the Prophet: to encourage the hearts of believers to cherish the hope of a better condition.

The Prophet says that God “called him one or alone,” not only because Abraham had no one with him when he was called out of his country, but also because the Lord allowed him to dwell in the land of Canaan without children until extreme old age. Consequently, he had no hope of having children, especially because Sarah (Genesis 16:2) was also barren. And when finally, as a solace for their childless condition, one son was given to them, not long afterward, it appeared he was to be led to slaughter. Yet the Lord increased and enriched him with a great number of children.

How necessary this consolation was to the Jews I noted earlier, and it can be easily learned from their condition, which history plainly shows was wretched and unhappy. For us also, in the present day, amidst this troubled condition of the Church, it is highly necessary, so that we may not be discouraged because our number is small, and so that we may hope that God will increase His Church by unexpected methods. We see a very clear and striking testimony of this in the blessing by which the Lord increased, even into extreme old age, the posterity of Abraham, who was childless and solitary. That promise relates not only to the Jews but also to other nations; and for this reason also he was no longer called Abram, but Abraham (Genesis 17:5).