The angel of the LORD encamps around those who fear Him, and delivers them.
If we have a promise, then it follows without fail that angels are round about us. This is why Psalm 34:7 states: The angel of the Lord encamps around those who fear Him." Likewise Psalm 91:11: "He will give His angels charge of you." This [Isaac and his family] believed firmly and without doubt. Therefore we, too, if we are godly, should believe in the promise of Him who cannot lie. Then we are surely under His protection, and it is also certain that angels are with us. But if any evil befalls us beyond, or contrary to, this trust and protection, this happens because of a special purpose that is hidden from us and especially from our adversaries. But the godly should comfort themselves in this manner: "I know that I have guardian angels; but that I have to bear some misfortune, this I leave to the will of God. For I am in the camp of the angels. God is not a liar. Therefore He will not forsake me."
From Lectures on Genesis (Luther's Works 5:62)