Corpus of Latin Ecclesiastical Writers, Vol. LXII: Ambrose's Exposition of Psalm 118

9. If anyone, therefore, walks in the way, let him search the testimonies of the Lord. Although this may be mystical, it nevertheless also has moral lessons within it, since he who searches the testimonies of the Lord can walk better in the way. For as some most intemperate person, captured by the heat of adulterous desire and overcome by lust or indulging in crimes, who does not wish to resist the appetite of his flesh, turning hither and thither, if by chance he sees no one, rushes into crime; the same person, however, although a ready occasion for obtaining it is present, investigates everything diligently, turns his eyes away from the justice of the curious, and is frequently anxious for his reputation; he blushes at the testimony of error, who does not blush at the error itself; and if by chance he recognizes some spectator of his crime, modesty sequesters his intemperance, and although he might attempt the temptations of lust with a maidservant or a common harlot, where there is no danger of being detected, he nevertheless abandons the attempt out of shame. How much more, if anyone would lift up the eyes of his mind and consider that all things are full of angels, the air, the lands, the sea, the churches, over which the angels preside—for the Lord sends his angels for the protection of those who are to be the heirs of heavenly promises—can he renounce a conceived sin! Whence comes this crowd of the innocent, if not from those sinners? The nature of all is the same, but the discipline is different. Circumcision is nothing and uncircumcision is nothing, but the observance of the commandments of God increases the grace of nature itself.

Reference: Romans 13:10; 1 Corinthians 7:19. The author discusses the omnipresence of angels and divine oversight as a deterrent to sin.