John Chrysostom

"For the Son of Man is come to save that which was lost." Again, He is putting another reason stronger than the former, and connects with it a parable, by which He brings in the Father also as desiring these things. ... And by the angels again that are entrusted with these same mean brethren, He makes them objects of veneration, and from His own will and passion (for when He said, "The Son of Man is come to save that which was lost," He signifies even the cross, like as Paul saith, speaking of a brother, "For whom Christ died"); and …

Homily on the Gospel of Matthew 59 ↗

Thomas Aquinas

But lest it seem to be a small thing that angels have been deputed to the guardianship of men, he proves this also by the ministry of Christ. And first he proves this; secondly, he introduces a similitude. He says therefore that little ones should not be despised, because the Son of man came to save that which was lost. 1 Tim 1:15: Christ Jesus came into this world to save sinners. Above, 1:21: for he shall save his people from their sins.

Commentary on Matthew ↗

Remigius of Rheims

By that which was lost, understand the human race; for all the elements have kept their place, but man was lost, because he has broken his ordained place.

Catena Aurea by Aquinas ↗