There is no branch of Christian archæology in which the labours of De Rossi have produced a more startling revolution than in the history of Christian art. When the Reformers of the sixteenth century protested against the use of pictures and images in churches, and boldly declared that it was contrary to the practice of primitive ages, Catholics had no monuments to appeal to in refutation of such a statement. The remains of the Christian literature of those times were scanty, and more or less silent upon the subject, and the paintings in the Catacombs were buried in darkness. Even when they were brought to light, it was not easy at once to speak positively as to their chronology. By and by, in proportion as the Catacombs were more explored, and it became possible to compare the paintings of one cemetery with those of another, and all of them with the paintings of Pompeii, the baths of Titus, the tomb of the Nasones, and other newly-recovered Pagan monuments whose ages were known, Catholic writers began to assume a bolder tone, and to claim a greater antiquity for the Christian use of painting than they had done before. Still it was impossible to fix any dates with precision so long as men confined themselves to the examination of single monuments, illustrating them perhaps with great learning, but without reference to the place where each monument had been found, the circumstances under which it had been executed, its connection with the history of the period, and the internal and external development of the Christian community at the time and place to which it belonged.