11 “O magnify the Lord our God!”. Again we magnify Him. He who is merciful even when He strikes, how is He to be praised, how is He to be magnified? Can you show this unto your son, and cannot God? For you are not good when thou dost caress your son, and evil when you strike him. Both when thou dost caress him you are a father, and when you strike him, you are his father: thou dost caress him, that he may not faint; you strike him, that he may not perish. “O magnify the Lord our God, and worship Him upon His holy hill: for the Lord our God is holy.”
As he said above, “O magnify the Lord our God and fall down before His footstool:” now we have understood what it is to worship His footstool: thus also but now after he had magnified the Lord our God, that no man might magnify Him apart from His hill, he has also praised His hill. What is His hill? We read elsewhere concerning this hill, that a stone was cut from the hill without hands, and shattered all the kingdoms of the earth, and the stone itself increased. This is the vision of Daniel which I am relating.
This stone which was cut from the hill without hands increased, and “became,” he says, “a great mountain, and filled the whole face of the earth.” Let us worship on that great mountain, if we desire to be heard. Heretics do not worship on that mountain, because it has filled the whole earth; they have stuck fast on part of it, and have lost the whole. If they acknowledge the Catholic Church, they will worship on this hill with us. For we already see how that stone that was cut from the mountain without hands has increased, and how great tracts of earth it has prevailed over, and unto what nations it has extended.
What is the mountain whence the stone was hewn without hands? The Jewish kingdom, in the first place; since they worshipped one God. Thence was hewn the stone, our Lord Jesus Christ....That stone then was born of the mountain without hands: it increased, and by its increase broke all the kingdoms of the earth. It has become a great mountain, and has filled the whole face of the earth. This is the Catholic Church, in whose communion rejoice that you are. But they who are not in her communion, since they worship and praise God apart from this same mountain, are not heard unto eternal life; although they may be heard unto certain temporal things.
Let them not flatter themselves, because God hears them in some things: for He hears Pagans also in some things. Do not the Pagans cry unto God, and it rains? Wherefore? Because He makes His sun to rise over the good and the bad, and sends rain upon the just and the unjust. Boast not therefore, Pagan, that when you cry unto God, God sends rain, for He sends rain upon the just and the unjust. He has heard you in temporal things: He hears you not in things eternal, unless you have worshipped in His holy hill. “Worship Him upon His holy hill: for the Lord our God is holy.”...
Source: The Enarrations, or Expositions, on the Psalms (New Advent)