Question Three of Chapter Four of the Second Decade
Why a guardian angel is assigned to a human being
It is asked: for what reason are angels assigned to the guardianship of human beings?
To this it is answered as follows. Angels are deputed to the guardianship of human beings for four causes.
The first is that they may be advanced and make progress in the good of grace.
The second is that they may not fall into the evil of guilt.
The third is that, if they fall, they may afterward rise again from the evil of guilt.
The fourth is that, if they do not rise again from the evil of guilt, they may not fall so often.
For no wayfarer, insofar as he is a wayfarer, lacks the mercy of God which consists in giving an angel to a human being as guardian for the aforesaid causes.
Take notice, however, that just as it belongs to divine providence to prevent evils in such a way that the condition of nature is nevertheless preserved, so also does this belong to the guardianship of angels, which is an execution of divine providence.
For it does not belong to angels, in guarding human beings, to compel them toward virtue and draw them back from vices, because in this way freedom of choice and the basis of merit would perish.
See Alexander, Second Part of the Summa, question 40, article 4, question 4.