qui custos es mei,
me, tibi commissum pietate superna,
rege et guberna.
This excerpt from a sermon by St. Bernard of Clairvaux (Sermo 12 in psalmum Qui habitat, 3. 6-8: Opera omnia, Edit. Cisterc. 4 , 458-462) is used in the Roman Office of Readings for the memorial of the Guardian Angels on October 2.
He has given his angels charge over you to guard you in all your ways. Let them thank the Lord for his mercy; his wonderful works are for the children of men. Let them give thanks and say among the nations, the Lord has done great things for them. O Lord, what is man that you have made yourself known to him, or why do you incline your heart to him? And you do incline your heart to him; you show him your care and your concern. Finally, you send your only Son and the grace of your Spirit, and promise him a vision of your countenance. And so, that nothing in heaven should be wanting in your concern for us, you send those blessed spirits to serve us, assigning them as our guardians and our teachers.
He has given his angels charge over you to guard you in all your ways. These words should fill you with respect, inspire devotion and instil confidence; respect for the presence of angels, devotion because of their loving service, and confidence because of their protection. And so the angels are here; they are at your side, they are with you, present on your behalf. They are here to protect you and to serve you. But even if it is God who has given them this charge, we must nonetheless be grateful to them for the great love with which they obey and come to help us in our great need.
So let us be devoted and grateful to such great protectors; let us return their love and honour them as much as we can and should. Yet all our love and honor must go to him, for it is from him that they receive all that makes them worthy of our love and respect.
We should then, my brothers, show our affection for the angels, for one day they will be our co-heirs just as here below they are our guardians and trustees appointed and set over us by the Father. We are God’s children although it does not seem so, because we are still but small children under guardians and trustees, and for the present little better than slaves.
Even though we are children and have a long, a very long and dangerous way to go, with such protectors what have we to fear? They who keep us in all our ways cannot be overpowered or led astray, much less lead us astray. They are loyal, prudent, powerful. Why then are we afraid? We have only to follow them, stay close to them, and we shall dwell under the protection of God’s heaven.