Émile Mâle says that medieval Christian art in its last period had lost touch with the great tradition of symbolism which had been so important in the twelfth and thirteenth centuries, and still largely dominated the art of the fourteenth. But there was one great symbolical idea which survived, and that was the harmony of the Old and New Testaments; and so we find among the most popular subjects of fifteenth-century Church art the concordance of the Apostles and Prophets in the Creed, and the series of parallels between the life of Jesus and episodes of Old Testament history, which were summed up and digested in the Biblia Pauperum and the Speculum Humanae Salvationis. The reason for the popularity of these subjects was, no doubt, their didactic value, and though Mâle does not develop this side of the subject, we may say that one, though not the only, characteristic of the religious art of the fifteenth century was that, instead of being symbolical, it became didactic. We find in this period a whole series of subjects which reduced the articles of Christian faith and practice to pictorial form, and seem to have been intended to illustrate the medieval catechism by which the teaching of the Church was imparted.