3 “Sing unto the Lord, bless His Name: be telling good tidings of His salvation from day to day”. How does the building increase? “Be telling,” he says, “good tidings of His salvation from day to day.” Let it be preached from day to day; from day to day, he says, let it be built; let My house, says God, increase. And as if it were said by the workmen, Where dost Thou command it to be built? Where do You will Your house to increase? Choose for us some level, spacious spot, if Thou wish an ample house built You.
Where dost Thou bid us be telling good tidings from day to day? He shows the place: “Declare His honour unto the heathen:” His honour, not yours. O you builders, “Declare His honour unto the heathen.” Should ye choose to declare your own honour, you shall fall: if His, you shall be built up, while you are building. Therefore they who choose to declare their own honour, have refused to dwell in that house; and therefore they sing not a new song with all the earth. For they do not share it with the whole round world; and hence they are not building in the house, but have erected a whited wall.
How sternly does God threaten the whited wall? There are innumerable testimonies of the Prophets, whence He curses the whited wall. What is the whited wall, save hypocrisy, that is, pretence? Without it is bright, within it is dirt....A certain person, speaking of this whited wall, said thus: “as, if in a wall which stands alone, and is not connected with any other walls, you make a door, whoever enters, is out of doors; so in that part which has refused to sing the new song together with the house, but has chosen to build a wall, and that a whited one, and not solid, what avails it that it has a door?”
If you enter, you are found to be without. For because they themselves did not enter by the door, their door also does not admit them within. For the Lord says, “I am the door: by Me they enter in.”...“Declare His honour unto the heathen.” What is, unto the heathen? Perhaps by nations but a few are meant: and that part which has raised the whited wall has still somewhat to say: why are not Getulia, Numidia, Mauritania, Byzacium, nations? Provinces are nations. Let the word of God take the word from hypocrisy, from the whited wall, building up the house over the whole world. It is not enough to say, “Declare His honour unto the heathen;” that you may not think any nations excepted, he adds, “and His wonders unto all people.”
Source: The Enarrations, or Expositions, on the Psalms (New Advent)