15 “In whatsoever day I shall have called upon You, behold I have known that my God are You”. A great knowledge. He says not, “I have known that God You are:” but, “that my God are You.” For yours He is, when you He succours: yours He is, when thou to Him art not an alien. Whence is said, “Blessed the people of whom is the Lord the God of the same.” Wherefore “of whom is”? For of whom is He not? Of all things indeed God He is: but of those men the God peculiarly He is said to be, that love Him, that hold Him, that possess Him, that worship Him, as though belonging to His own House: the great family of Him are they, redeemed by the great blood of the Only Son. How great a thing has God given to us, that His own we should be, and He should be ours! But in truth foreigners afar have been put from holy men, sons alien they are. See what of them is said in another Psalm: “O Lord, deliver me,” he says, “from the hand of alien sons, of whom the mouth has spoken vanity, and the right hand of them is a right hand of iniquity.”...
Source: The Enarrations, or Expositions, on the Psalms (New Advent)