Calvin's commentary on Romans 9.8

Calvin, commenting on Romans 9.6-8, "He had indeed before briefly noticed, that there was a difference between the natural children of Abraham, that though all were adopted by circumcision into a participation of the covenant, yet the grace of God was not effectual in them all; and hence that they, who enjoy the favor of God, are the children of the promise. But how it thus happened, he has been either silent or has obscurely hinted. Now indeed he openly ascribes the whole cause to the election of God, and that gratuitous, and in no way depending on men; so that in the salvation of the godly nothing higher (nihil superius) must be sought than the goodness of God, and nothing higher in the perdition of the reprobate than his just severity.Calvin's Commentaries.

Romans 9:6-8 (ESV)

^6 But it is not as though the word of God has failed. For not all who are descended from Israel belong to Israel,

^7 and not all are children of Abraham because they are his offspring, but “Through Isaac shall your offspring be named.”

^8 This means that it is not the children of the flesh who are the children of God, but the children of the promise are counted as offspring.