John Calvin Commentary Genesis 11:27

"Now these are the generations of Terah. Terah begat Abram, Nahor, and Haran. And Haran begat Lot." — Genesis 11:27 (ASV)

Terah begat Abram. Here also Abram is placed first among his brothers, not (as I suppose) because he was the firstborn, but because Moses, intent on the scope of his history, was not very careful in the arrangement of the sons of Terah. It is also possible that he had other sons.

The reason why Moses speaks especially of them is obvious: namely, on account of Lot, and of the wives of Isaac and Jacob. I will now briefly state why I think Abram was not the firstborn. Moses shortly afterwards says that Haran died in his own country, before his father left Chaldea and went to Haran. But Abram was seventy-five years old when he departed from Haran to dwell in the land of Canaan. And this number of seventy-five years is expressly given after the death of Terah.

Now, if we suppose that Abram was born in his father’s seventieth year, we must also allow that sixty years of Terah’s age would be lost, which is most absurd. Luther's conjecture that God buried that time in oblivion to hide the end of the world from us is, firstly, frivolous, and secondly, can be refuted by solid and convincing arguments.

Others violently wrest the words to apply them to a former departure and think that he lived with his father at Haran for sixty years, which is most improbable. For what purpose would they have protracted their stay so long in the middle of their journey? But there is no need for laborious discussion.

Moses is silent regarding the age of Abraham when he left his own country, but says that in the seventy-fifth year of his age, he came into the land of Canaan when his father, having reached the two hundred and fifth year of his life, had died. Who will not, therefore, infer that he was born when his father had attained his one hundred and thirtieth year? But he is named first among those sons whom Terah is said to have begotten when he himself was seventy years old.

I grant it; but this order of mention does nothing to prove the order of birth, as we have already said. Nor, indeed, does Moses declare in what year of his life Terah begat sons, but only that he had passed the above age before he begat the three sons mentioned here.

Therefore, Abraham's age is to be ascertained by another method of computation: namely, from the fact that Moses assigns to him the age of seventy-five when his father died, whose life had reached two hundred and five years. A firm and valid argument is also deduced from the age of Sarai.

It appears that she was not more than ten years younger than Abraham. If she was the daughter of his younger brother, she would necessarily have equalled her own father in age. Those who raise an objection, to the effect that she was Nahor's daughter-in-law or only his adopted daughter, produce nothing beyond a sheer cavil.