A guardian angel is an angel assigned to protect and guide a particular person, defending from evil, suggesting good thoughts and wise counsels and helping in prayer. Belief in guardian angels can be traced throughout all antiquity in countries around the world. Greeks and Romans believed that each individual was under the special protection of a spirit who prompted him to good deeds, warned him against danger and guided him through life. Guardian angels are also believed to protect cities and countries.
The Jews of the Old Testament had a belief in guardian angels (see: Barclay, Belief in Angels Among the Jews). In Exodus 32:34, for example, God says to Moses "Now go, lead the people to the place I spoke of, and my angel will go before you." (NIV) Likewise, we see guardian angels in the New Testament; one example is in Matthew 18:10, where Jesus is quoted as saying "See that you do not look down on one of these little ones. For I tell you that their angels in heaven always see the face of my Father in heaven." (NIV)
The Catholic Church has never defined the belief that every individual soul has a guardian angel, and it is therefore not an article of faith. However, as St. Jerome expressed it: "how great the dignity of the soul, since each one has from his birth an angel commissioned to guard it." (Comm. in Matt., xviii, lib. II).
The concept of tutelary angels and their hierarchy was extensively developed in Christianity in the 5th century by Pseudo-Dionysius the Areopagite in his work De Coelesti Hierarchia (Celestial Hierarchy). It's been written that Thomas Aquinas (Summa Theologica, I.108) follows the Hierarchia (6.7) in dividing the angels into three hierarchies each of which contains three orders, based on their proximity to God, corresponding to the nine orders of angels recognized by St. Gregory Dialogus:
Seraphim, Cherubim, and Thrones;
Dominations, Virtues, and Powers;
Principalities, Archangels, and Angels.
These categories were influenced by two passages from the New Testament in particular:
Ephesians 1:19-21: "... His incomparably great power ... is like the working of His mighty strength, which He exerted in Christ when He raised him from the dead and seated Him at His right hand in the heavenly realms, far above all rule and authority, power and dominion, and every title that can be given, not only in the present age but also in the one to come."
Colossians 1:16: "For by him all things were created: things in heaven and on earth, visible and invisible, whether thrones or powers or rulers or authorities; all things were created by him and for him."
Considerable work was also done by Honorius of Autum who created a systemized view of individual guardian angels.
For more information, see:
"Feast of the Guardian Angels," Catholic Encyclopedia, 1911, at New Advent <http://www.newadvent.org/cathen/07050a.htm>
Jaroslav Pelikan, The Christian Tradition: A History of the Development of Doctrine, Vol. 1, p. 297.
"Guardian Angel" and "Guardian Spirit," Encyclopedia Americana, Volume 13, (1919).
The Guardian Angel (London: Joseph Masters, 1858), a collection of poems and sketches concerning Guardian Angels.
Songs, Poems and Notes concerning Guardian Angels include:
Feast of The Guardian Angels, October 2.
The Angels Song (Version 2 of Last Night As I Lay Sleeping)
The Boy's Dream (Last night as I was laid and slept)
Source: "Night," The Guardian Angel (London: Joseph Masters, 1858).