Theophylact of Ohrid

And when they see this bread, it is their obligation to proclaim it to others as well, just as the shepherds, having seen the Infant, told others about Him.

Commentary on Luke ↗

Ancient Greek Expositor

Beholding with hidden faith indeed the happy events which had been told them, and not content with marvelling at the reality of those things which at the very first they saw and embraced when the Angel told them, they began to relate them not only to Mary and Joseph, but to the others also, (and what is more they impressed them on their minds,) as it follows, And all who heard it marvelled. …

Catena Aurea by Aquinas ↗

Bede

The shepherds did not keep silent about the hidden mysteries that they had come to know by divine influence. They told whomever they could. Spiritual shepherds in the church are appointed especially for this, that they may proclaim the mysteries of the Word of God and that they may show to their listeners that the marvels which they have learned in the Scriptures are to be marveled at.

Homilies on the Gospels 1.7 ↗

Bonaventure

And because this sign was given by the Angel, therefore it is added: But seeing, they recognized concerning the word that had been spoken to them about this child, namely, that it was true. Whence they could now say that word of the Samaritans to the Angel himself: "We no longer believe because of your speech, for we ourselves have heard and we know that this is truly the Savior of the world." Truly they recognized, because they were of the sheep of that Shepherd, who says in John chapter ten: "I know Mine, and Mine know Me." This …

Commentary on Luke, Chapter 2 ↗