16 “You have redeemed in Your arm Your people”. “With Your arm,” that is, with Your power. “And to whom has the arm of the Lord been revealed?” “You have redeemed in Your arm Your people, the sons of Israel and of Joseph.” How as if two peoples, “the sons of Israel and of Joseph”? Are not the sons of Joseph among the sons of Israel?...He has admonished us of some distinction to be made. Let us search out our spirit, perchance God has placed there something— God whom we ought even by night to seek with our hands, in order that we may not be deceived— perchance we shall discover even ourselves in this distinction of “sons of Israel and of Joseph.”
By Joseph He has willed another people to be understood, has willed that the people of the Gentiles be understood. Why the people of the Gentiles by Joseph? Because Joseph was sold into Egypt by his brethren. That Joseph whom the brethren envied, and sold him into Egypt, when sold into Egypt, toiled, was humbled; when made known and exalted, flourished, reigned. And by all these things he has signified what? What but Christ sold by His brethren, banished from His own land, as it were into the Egypt of the Gentiles?
There at first humbled, when the Martyrs were suffering persecutions: now exalted, as we see; inasmuch as there has been fulfilled in Him, “There shall adore Him all kinds of the earth, all nations shall serve Him.” Therefore Joseph is the people of the Gentiles, but Israel the people of the Hebrew nation. God has redeemed His people, “the sons of Israel and of Joseph.” By means of what? By means of the corner stone, wherein the two walls have been joined together.
Source: The Enarrations, or Expositions, on the Psalms (New Advent)