William Burkitt on John 1:13

Because the bragging Jews did much boast of their natural birth and descent from Abraham, as being his blood and offspring, therefore it is here asserted, that men become not the children of God by natural propagation, but by spiritual regeneration: They are not born of blood. Grace runs not in the blood, piety is not hereditary. Religious parents propagate corruption, not regeneration. Were the conveyances of grace natural, good parents would not be so ill-suited with children as sometimes they are. No person then whatsoever has the gracious privilege of adoption by the first birth. They are not born of blood, nor of the will of the flesh, nor of the will of man; that is, no man by the utmost improvement of nature can raise himself up to this privilege of adoption, and be the author and efficient cause of his own regeneration.

Learn hence, That man in all his capacities is too weak to produce the work of regeneration in himself. They, says Dr. Hammond, who by the influence of the highest rational principles, live most exactly according to the rule of rational nature, that is, of unregenerated morality, are the persons here described.

Learn, 2. That God alone is the prime efficient Cause of regeneration. He works upon the understanding by illumination, and upon the will by sanctification; Which were born, not of blood, nor of the will of the flesh, nor of the will of man, but of God.