John Calvin Commentary Romans 9:10

"And not only so; but Rebecca also having conceived by one, [even] by our father Isaac--" — Romans 9:10 (ASV)

And not only, etc. There are in this chapter some broken sentences, such as this is — But Rebecca also, who had conceived by one, our father Isaac; for he leaves off in the middle, before he comes to the principal verb. The meaning, however, is, that the difference regarding the possession of the promise may not only be seen in the children of Abraham, but that there is a much more evident example in Jacob and Esau: for in the former instance some might allege that their condition was unequal, the one being the son of a female servant; but these were of the same mother, and were even twins: yet one was rejected, and the other was chosen by the Lord. It is therefore clear, that the fulfilment of the promise does not take place in all the children of the flesh indiscriminately.

And as Paul refers to the persons to whom God made known his purpose, I prefer to understand that a masculine pronoun is implied, rather than a neuter, as Erasmus has done: for the meaning is, that God’s special election had not been revealed only to Abraham, but also to Rebecca, when she gave birth to her twins.