6 What is it, most beloved? You who know yourselves to be Israelites according to Abraham's seed, you who are of the house of Jacob, heirs according to promise, know that even you have gone forth from Egypt, since you have renounced this world; that you have gone forth from a foreign people, since by the confession of piety, you have separated yourselves from the blasphemies of the Gentiles. For it is not your tongue, but a foreign one, which knows not how to praise God, to whom you sing Allelujah. For “Judah” has become “His sanctuary” in you; for “he is not a Jew which is one outwardly; neither is that circumcision, which is outward in the flesh; but he is a Jew which is one inwardly, and by circumcision of the heart.” Examine then your hearts, if faith has circumcised them, if confession has cleansed them; in you “Judah” has become “His sanctuary,” in you “Israel” has become “His dominion.” For “He gave” unto you “the power to become the sons of God.”...
Source: The Enarrations, or Expositions, on the Psalms (New Advent)