That every individual soul has a guardian angel has never been defined by the Church, and is, consequently, not an article of faith; but it is the “mind of the Church”, 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.”
In the Bible this doctrine is clearly discernible and its development is well marked. In Genesis 28-29, angels not only act as the executors of God’s wrath against the cities of the plain, but they deliver Lot from danger; in Exodus 12-13, an angel is the appointed leader of the host of Israel, and in 32:34, God says 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 90:11: “For he hath given his angels charge over thee; to keep thee in all thy ways.” (Cf. Psalm 33:8 and 34:5.) Lastly, in Daniel 10 angels are entrusted with the care of particular districts; one is called “prince of the kingdom of the Persians”, and Michael is termed “one of the chief princes”; cf. Deuteronomy 32:8; and Ecclesiasticus 17:17.
This sums up the Old Testament doctrine on the point; it is clear that the Old Testament conceived of God’s angels as His ministers who carried out his behests, and who were at times given special commissions, regarding men and mundane affairs. There is no special teaching; the doctrine is rather taken for granted than expressly laid down; cf. 2 Machabees 3:25; 10:29; 11:6; 15:23.
But in the New Testament the doctrine is stated with greater precision. Angels are everywhere the intermediaries between God and man; and Christ set a seal upon the Old Testament teaching: “See that you despise not one of these little ones: for I say to you, that their angels in heaven always see the face of my Father who is in heaven.” (Matthew 18:10). A twofold aspect of the doctrine is here put before us: even little children have guardian angels, and these same angels lose not the vision of God by the fact that they have a mission to fulfil on earth.
Without dwelling on the various passages in the New Testament where the doctrine of guardian angels is suggested, it may suffice to mention the angel who succoured Christ in the garden, and the angel who delivered St. Peter from prison. Hebrews 1:14 puts the doctrine in its clearest light: “Are they not all ministering spirits, sent to minister for them, who shall receive the inheritance of salvation?” This is the function of the guardian angels; they are to lead us, if we wish it, to the Kingdom of Heaven.
(Principal source – Catholic Encyclopedia – 1913 edition )