If, therefore, He made them "His mother and His brethren" who were not so, how could He deny them these relationships who really had them? Surely only on the condition of their deserts, and not by any disavowal of His near relatives; teaching them by His own actual example, that "whosoever preferred father or mother or brethren to the Word of God, was not a disciple worthy of Him." Besides, His admission of His mother and His brethren was the more express, from the fact of His unwillingness to acknowledge them. That He adopted others only confirmed those in their relationship to Him whom He refused because of their offence, and for whom He substituted the others, not as being truer relatives, but worthier ones. Finally, it was no great matter if He did prefer to kindred (that) faith which it did not possess.
Source: Against Marcion Book IV