39 Again, when it is said that the Church is the sister of Christ, not by the mother but by the father, we learn the excellence of the relation, which is not of the temporary nature of earthly descent, but of divine grace, which is everlasting. By this grace we shall no longer be a race of mortals when we receive power to be called and to become sons of God. This grace we obtain not from the synagogue, which is the mother of Christ after the flesh, but from God the Father.
And when Christ calls us into another life where there is no death, He teaches us, instead of acknowledging, to deny the earthly relationship, where death soon follows upon birth; for He says to His disciples, "Call no man your father upon earth; for you have one Father, who is in heaven." And He set us an example of this when He said, "Who is my mother, and who are my brethren? And stretching forth His hand to His disciples, He said, These are my brethren." And lest any one should think that He referred to an earthly relationship, He added, "Whosoever shall do the will of my Father, the same is my brother, and sister, and mother;" as much as to say, I derive this relationship from God my Father, not from the Synagogue my mother; I call you to eternal life, where I have an immortal birth, not to earthly life, for to call you away from this life I have taken mortality.
Source: Reply to Faustus the Manichaean (New Advent)