Bede

And this will be a sign for you. You will find a baby wrapped in swaddling clothes and lying in a manger. The infancy of the Savior and the proclamations of angels and the testimonies of the evangelists have often been impressed upon us, so that it might be firmly fixed in our hearts what He became for us. For He was wounded for our iniquities, and He was made weak for our sins (Isaiah 53). …

On the Gospel of Luke ↗

Bonaventure

And this shall be a sign to you, etc. After the disposition preparing for hearing and the announcement inviting to exultation, there follows here thirdly a confirmation assuring for believing. And since in the mouth of two witnesses every word must stand, therefore the shepherds are confirmed in a twofold manner: through sight and hearing; for these two senses above all serve us for knowing. First, therefore, they are assured through a probable sign, then through a credible testimony. …

Commentary on Luke, Chapter 2 ↗

Maximus of Turin

But if perhaps the swaddling clothes are mean in thy eyes, admire the Angels singing praises together. If thou despisest the manger, raise thy eyes a little, and behold the new star in heaven proclaiming to the world the Lord's nativity. If thou believest the mean things, believe also the mighty. If thou disputest about those which betoken His lowliness, look with reverence on what is high and heavenly.

Catena Aurea by Aquinas ↗