St. Ambrose: On guardian angels

“The servants of Christ are protected by invisible, rather than visible, beings. But if these guard you, they do so because they have been summoned by your prayers.”

~St. Ambrose (c. 340 – 397): Sermo contra Auxent., 11.

Linaioli Tabernacle (detail), by Fra Angelico.
Tempera on wood, c. 1433;
Museo di San Marco, Florence.