Gregory the Great
Catena Aurea · Matthäusevangelium 18
Take heed that ye despise not one of these little ones; for I say unto you, That in heaven their angels do always behold the face of my Father which is in heaven. For the Son of man is come to save that which was lost. How think ye? if a man have an hundred sheep, and one of them be gone astray, doth he not leave the ninety and nine, and goeth into the mountains, and seeketh that which is gone astray? And if so be that he find it, verily I say unto you, he rejoiceth more of that sheep, than of the ninety and nine which went not astray. Even so it is not the will of your Father which is in heaven, that one of these little ones should perish.
(Hom. in Ev. 34. 12) But Dionysius says, that it is from the ranks of the lesser Angels that these are sent to perform this ministry, either visibly or invisibly, for that those higher ranks have not the employment of an outward ministry.