John Lightfoot on John 1:13

[Which were born, not of blood.] It may be a question here, whether the evangelist in this place opposeth regeneration to natural generation, or only to those ways by which the Jews fancied men were made the sons of God. Expositors treat largely of the former: let us a little consider the latter.

I. Not of bloods. Observe the plural number: "Our Rabbins say, That all Israel had thrown off circumcision in Egypt--but at length they were circumcised, and the blood of the passover was mingled with the blood of the circumcised, and God accepted every one of them and kissed them." "I said, while thou wert in thy bloods, Live: i.e. in the twofold blood, that of the passover, and that of the circumcision." The Israelites were brought into covenant by three things; by circumcision, by washing, and by offering of sacrifices. In the same manner, a heathen, if he would be admitted into covenant, he must of necessity be circumcised, baptized, and offer sacrifice. We see how of bloods of the passover and circumcision, they say the Israelites were recovered from the degeneracy: and how of the bloods of circumcision and sacrifices (with the addition only of washing), they supposed the Gentiles might become the sons of God, being by their proselytism made Israelites, and the children of the covenant: for they knew of no other adoption or sonship.

II. Of the will of the flesh. In the same sense wherein the patriarchs and other Jews were ambitious by many wives to multiply children of themselves, as being of the seed of Israel and children of the covenant.

III. Of the will of man, in that sense wherein they coveted so many proselytes, to admit them into the religion of the Jews, and so into covenant and sonship with God.

These were the ways by which the Jews thought any became the sons of God, that is, by being made Israelites. But it is far otherwise in the adoption and sonship that accrues to us by the gospel.