16 But who is this whom we speak of? “The Stone which the builders rejected”; for “It has become the head Stone of the corner” to “make in Himself of two one new man, so making peace; and that He might reconcile both unto God in one body;” circumcision, to wit, and uncircumcision.
17. “By the Lord was it made unto it”: that is, it is made into the head stone of the corner by the Lord. For although He would not have become this, had He not suffered: yet He became not this through those from whom He suffered. For they who were building, refused Him: but in the edifice which the Lord was secretly raising, that was made the head stone of the corner which they rejected. “And it is marvellous in our eyes:” in the eyes of the inner man, in the eyes of those that believe, those that hope, those that love; not in the carnal eyes of those who, through scorning Him as if He were a man, rejected Him.
18. “This is the day which the Lord has made”. This man remembers that he had said in former Psalms, “Since He has inclined His ear unto me, therefore will I call upon Him as long as I live;” making mention of his old days; whence he now says, “This is the day which the Lord has made;” that is, wherein He has given me Salvation. This is the day whereof He said, “In an acceptable time have I heard you, and in a day of Salvation have I helped you;” that is, a day wherein He, the Mediator, has become the head Stone of the corner. “Let us rejoice,” therefore, “and be glad in Him.”
19. “Save me now, O Lord: prosper Thou well my way, O Lord”. Because it is the day of Salvation, “save me:” because we, returning from a long pilgrimage, are separated from those who hated peace, with whom we were peaceful, and who, when we spoke to them, made war upon us without a cause; “prosper well our way” as we return, since You have become our Way.
20. “Blessed be He that comes in the Name of the Lord”. Cursed, therefore, is he that comes in his own name; as He says in the Gospel: “if another shall come in his own name, him you will receive.” “We have blessed you out of the house of God.” I believe that these are the words of the great to the little, of those great ones, to wit, who in spirit commune with God the Word, who is with God, as they may in this life; and yet temper their discourse for the sake of the little ones, so that they may sincerely say what the Apostle says: “For whether we be beside ourselves, it is to God: or whether we be sober, it is for your cause. For the love of Christ constrains us.” They bless the little children from the inner house of the Lord, where that praise fails not age after age: consider therefore what they proclaim from thence.
21. “God is the Lord, who has showed us light”. That Lord, who came in the Lord's Name, whom the builders refused, and who became the head Stone of the corner, that “Mediator between God and man, Jesus Christ,” is God, He is equal with the Father, He has showed us light, that we might understand what we believed, and declare it to you who understand it not as yet, but already believe it. But that you also may understand, “Declare a holy day in full assemblies, even unto the horns of the altar;” that is, even unto the inner house of God, from which we have blessed you, where are the high places of the altar. “Declare a holy day,” not in a slothful manner, but “in full assemblies”. For this is the voice of joyfulness among those that keep holy day, who walk “in the place of the wonderful tabernacle, even unto the house of God.” For if there be there the spiritual sacrifice, the everlasting sacrifice of praise, both the Priest is everlasting, and the peaceful mind of the righteous an everlasting altar....
Source: The Enarrations, or Expositions, on the Psalms (New Advent)