But, as their name indicates the angels are also messengers of divine commands (Offertory). The holy Fathers teach that they preside over all created things, and St. Paul declares "that their mission is to serve the future heirs of salvation". (Hebrews 1:14.) This is why they are called guardian angels (Collect). It is generally thought that kingdoms, provinces, families, dioceses, churches, religious communities each have their protecting angel. In some countries they celebrate the feast of the angel of the nation. St. Francis of Hales says that the bishop as such has another guardian angel. Ezechiel says that the angel of the Temple of Jerusalem had a writing case hanging from his girdle (Ezech. 11). The angels of the churches, adds St. Basil, write down our irreverences.
That there is one for each just man, there can be no doubt, and the Church applies to the souls that are under the new Law what is written about the people under the Old Law. "I shall send My angel to march before you, to guard you on the way and to bring you into the land which I have prepared for you" (Epistle). Our guardian angel has for his mission to protect us and defend us in order that under his protection, safe from the snares of the enemies of our souls and from all adversities, we may reach the promised land of eternal life (Collect, Secret, Postcommunion). This faithful companion deserves our gratitude and the veneration due to one who already enjoys the beatific vision (Gospel). It is to encourage us to the habitual practice of this devotion that the Church instituted the feast of the Guardian Angels.
Benedicite Dominum, omnes Angeli ejus: potentes virtute, qui facitis verbum ejus, ad audiendam vocem sermonum ejus. * Benedic, anima mea, Domino: et omnia, quae intra me sunt, nomini sancto ejus.
Bless the Lord all ye His angels: you that are mighty in strength, and execute His word, hearkening to the voice of all His orders. * Bless the Lord, O my soul; and let all that is within me praise His holy name.
(Psalm 102:20,1 from the Introit of Mass)
Deus, qui ineffabili providentia sanctos Angelos tuos ad nostram custodiam mittere dignaris: largire supplicibus tuis; et eorum semper protectione defendi et aeterna societate gaudere.
O God, who in Thine ineffable providence, hast sent Thy holy angels to watch over us: grant, we humbly pray, that we may always be defended by their protection and may rejoice in their fellowship for evermore.
From the Catholic Encyclopaedia: http://www.newadvent.org/cathen/07050a.htm