The Birth of Jesus - Commentary on Luke 2

Fear not - The design of this exhortation is to alleviate their fear.   For, though it is profitable for the minds of men to be struck with   awe, that they may learn to "give unto the Lord the glory due unto his   name," (Psalm 29:2;) yet they have need, at the same time, of   consolation, that they may not be altogether overwhelmed. For the   majesty of God could not but swallow up the whole world, if there were   not some mildness to mitigate the terror which it brings. And so the   reprobate fall down lifeless at the sight of God, because he appears to   them in no other character than that of a judge. But to revive the   minds of the shepherds, the angel declares that he was sent to them for   a different purpose, to announce to them the mercy of God.