A guardian angel is an angel that is assigned to protect and guide a particular person, group, kingdom, or country. Belief in guardian angels can be traced throughout all antiquity. The concept of tutelary angels and their hierarchy was extensively developed in Christianity in the 5th century by Pseudo-Dionysius the Areopagite.
The theology of angels and tutelary spirits has undergone many refinements since the 5th century. Belief in both the East and the West is that guardian angels serve to protect whichever person God assigns them to, and present prayer to God on that person's behalf.
The belief in guardian angels can be traced throughout all antiquity; pagans such as Menander and Plutarch and Neo-platonists such as Plotinus held it.
The guardian angel concept is present in the books of the Hebrew Bible and the Old Testament, and its development is well marked. These books described God's angels as his ministers who carried out his behests, and who were at times given special commissions, regarding men and mundane affairs.
In Genesis 18-19, angels not only acted as the executors of God's wrath against the cities of the plain, but they delivered Lot from danger; in Exodus 32:34, God said to Moses: "my angel shall go before thee." At a much later period, we have the story of Tobias, which might serve for a commentary on the words of Psalm 91:11: "For he will command his angels concerning you to guard you in all your ways;" (Cf. Psalm 33:8 and 34:5)
The belief that angels can be guides and intercessors for men can be found in Job 33:23-6, and in Daniel 10:13 angels seem to be assigned to certain countries. In this latter case, the "prince of the kingdom of Persia" contends with Gabriel. The same verse mentions "Michael, one of the chief princes".