"Guardian Angel"

“GOD'S universal providence works through secondary causes…. The world of pure spirits stretches between the Divine Nature and the world of human beings; because Divine Wisdom has ordered that the higher should look after the lower, angels execute the divine plan for human salvation: they are our guardians, who free us when hindered and help to bring us home.”

—Commentary on the Sentences, 2, 11, 1, 1. 

"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."

—Summa Theologica, I, Q. 113, Art. 4.

Guardian Angel, by Pietro da Cortana. 

Oil on canvas, 1656; Galleria Nazionale d'Arte Antica.