See in what manner the angels announce to the shepherds the Prince of Shepherds: as a new-born Lamb, seen in a cave.
With frequent heralding of angels, and with manifold testimonies of the Gospels, the Infancy of the Savior is impressed on our hearts, so that deep within may be implanted the remembrance of what He has done for us.
And let us note that the sign given to us of the Savior is not that of a Child reclining in Tyrrhenian purple, but of one wrapped in poor swaddling clothes; not of One resting on a gilded bed, but lying in a manger.
-St. Bede the Venerable
But if these swaddling clothes seem to you unbecoming, look up and hear the heavenly choir. Should you deplore the manger, raise your eyes a little and see the new star in the sky, proclaiming to the world the Birth of the Lord. If you believe these unworthy, believe that these are wondrous. If you do not hold with the things that savor of lowliness, worship those that are sublime and heavenly.
That an angel appears to the watching shepherds, and that the Brightness of God shone round about them, may also be understood in a mystical sense; and this is: that they above others merit to behold the sublime mysteries, who know how to guard carefully their believing flocks. While they dutifully keep guard over their flocks, the divine grace will shine abundantly on them.
Collected by St. Thomas Aquinas in “Catena Aurea”