2 But that which is the house of God is also a city. For the house of God is the people of God; for the house of God is the temple of God....This is Jerusalem: she has guards: as she has builders, labouring at her building up, so also has she guards. To this guardianship these words of the Apostle relate: “I fear, lest by any means your minds should be corrupted from the simplicity which is in Christ.” He was guarding the Church. He kept watch, to the utmost of his power, over those over whom he was set.
The Bishops also do this. For a higher place was for this reason given the Bishops, that they might be themselves the superintendents and as it were the guardians of the people. For the Greek word Episcopus, and the vernacular Superintendent, are the same; for the Bishop superintends, in that he looks over. As a higher place is assigned to the vinedresser in the charge of the vineyard, so also to the Bishops a more exalted station is alloted. And a perilous account is rendered of this high station, except we stand here with a heart that causes us to stand beneath your feet in humility, and pray for you, that He who knows your minds may be Himself your keeper.
Since we can see you both coming in and going out; but we are so unable to see what are the thoughts of your hearts, that we cannot even see what ye do in your houses. How then can we guard you? As men: as far as we are able, as far as we have received power. And because we guard you like men, and cannot guard you perfectly, shall you therefore remain without a keeper? Far be it! For where is He of whom it is said, “Except the Lord keep the city, the watchman wakes but in vain?”.
We are watchful on our guard, but vain in our watchfulness, except He who sees your thoughts guard you. He keeps guard while you are awake, He keeps guard also while you are asleep. For He has once slept on the Cross, and has risen again; He no longer sleeps. Be Israel: for “the Keeper of Israel neither sleeps nor slumbers.” Yea, brethren, if we wish to be kept beneath the shadow of God's wings, let us be Israel. For we guard you in our office of stewards; but we wish to be guarded together with you. We are as it were shepherds unto you; but beneath that Shepherd we are fellow-sheep with you. We are as it were your teachers from this station; but beneath Him, the One Master, we are schoolfellows with you in this school.
Source: The Enarrations, or Expositions, on the Psalms (New Advent)