Why are angels called God's secret agents?

This term became popular from a book written by Billy Graham wherein he writes about how angels are our protectors essentially working undercover for God carrying out various missions. "The angels are the dispensers and administrators of the divine beneficence toward us; they regard our safety, undertake our defense, direct our ways, and exercise a constant solicitude that no evil befall us" --John Calvin" (Angels God's Secret Agents, Billy Graham, 1986, introductory page).