Whether the angel guardian ever forsakes a man?... It would seem that the angel guardian sometimes forsakes the man whom he is appointed to guard... On the contrary, The demons are ever assailing us, according to 1 Peter 5:8: "Your adversary the devil, as a roaring lion, goeth about, seeking whom he may devour." Much more therefore do the good angels ever guard us... the guardianship of the angels is an effect of Divine providence in regard to man. Now it is evident that neither man, nor anything at all, is entirely withdrawn from the providence of God: for in as far as a thing participates being, so far is it subject to the providence that extends over all being.

I, q. 113, art. 6 - Summa Theologica (1265–1274)