Objection 1. It would seem that angels are not appointed to the guardianship of all men. For it is written of Christ (Philippians 2:7) that "He was made in the likeness of men, and in habit found as a man." If therefore angels are appointed to the guardianship of all men, Christ also would have had an angel guardian. But this is unseemly, for Christ is greater than all the angels. Therefore angels are not appointed to the guardianship of all men.
Objection 2. Further, Adam was the first of all men. But it was not fitting that he should have an angel guardian, at least in the state of innocence: for then he was not beset by any dangers. Therefore angels are not appointed to the guardianship of all men.
Objection 3. Further, angels are appointed to the guardianship of men, that they may take them by the hand and guide them to eternal life, encourage them to good works, and protect them against the assaults of the demons. But men who are foreknown to damnation, never attain to eternal life. Infidels, also, though at times they perform good works, do not perform them well, for they have not a right intention: for "faith directs the intention" as Augustine says (Enarr. ii in Ps. 31). Moreover, the coming of Antichrist will be "according to the working of Satan," as it is written (2 Thessalonians 2:9). Therefore angels are not deputed to the guardianship of all men.
On the contrary, is the authority of Jerome quoted above (Article 2), for he says that "each soul has an angel appointed to guard it."
I answer that, Man while in this state of life, is, as it were, on a road by which he should journey towards heaven. On this road man is threatened by many dangers both from within and from without, according to Psalm 141:4: "In this way wherein I walked, they have hidden a snare for me." And therefore as guardians are appointed for men who have to pass by an unsafe road, so an angel guardian is assigned to each man as long as he is a wayfarer. When, however, he arrives at the end of life he no longer has a guardian angel; but in the kingdom he will have an angel to reign with him, in hell a demon to punish him.
Reply to Objection 1. Christ as man was guided immediately by the Word of God: wherefore He needed not be guarded by an angel. Again as regards His soul, He was a comprehensor, although in regard to His passible body, He was a wayfarer. In this latter respect it was right that He should have not a guardian angel as superior to Him, but a ministering angel as inferior to Him. Whence it is written (Matthew 4:11) that "angels came and ministered to Him."
Reply to Objection 2. In the state of innocence man was not threatened by any peril from within: because within him all was well ordered, as we have said above (95, 1,3). But peril threatened from without on account of the snares of the demons; as was proved by the event. For this reason he needed a guardian angel.
Reply to Objection 3. Just as the foreknown, the infidels, and even Anti-christ, are not deprived of the interior help of natural reason; so neither are they deprived of that exterior help granted by God to the whole human race--namely the guardianship of the angels. And although the help which they receive therefrom does not result in their deserving eternal life by good works, it does nevertheless conduce to their being protected from certain evils which would hurt both themselves and others. For even the demons are held off by the good angels, lest they hurt as much as they would. In like manner Antichrist will not do as much harm as he would wish.
Source: Summa Theologica (New Advent)