Who builds the Church and guards it?

St. Ambrose says, “As for the motive of this baptism of the Lord, the Lord Himself explains it in these words: ‘permit it to be now, for thus it is fitting for us to fulfill all righteousness’ (Matt. 3:15). God having done so much, by a divine favor, that, for the construction of his Church, after the patriarchs, the prophets, the angels, the only Son of God descended and come to baptism, do we not recognize with what truth How divinely has it been said of the Church: ‘Unless the Lord builds the house, they labor in vain who build it’ (Ps. 127:1). And nothing surprising that man cannot build, since he cannot keep: ‘Unless the Lord guards the city the watchman stays a wake in vain.’ Thus speaks some psalm (Ps. 126:1). I dare say, however, that man can not engage in a way unless he has the Lord to go before him; so it is written, ‘You shall walk after the Lord your God and fear Him’ (Deut. 13:4).”⁹⁰⁸