The thirteenth and the seventeenth centuries are the two noblest and fairest epochs of French Catholicism. The sixteenth century and the beginning of the seventeenth are the two noblest and fairest periods of French Protestantism. Among French Catholics I have chosen St. Louis in the thirteenth century and St. Vincent de Paul in the seventeenth, as two great and noble Christians, two earnest and illustrious representatives of the Christian faith and life, as well as of the loftiest thought and purest morality of their country and their generation. Among the Protestants of the sixteenth century, Calvin and Du Plessis Mornay present the same characteristics, and deserve an equal glory. These four men were emphatically and first of all Christians, in thought and life. Christian faith and piety shone out in all of them, notwithstanding their profound divergence and their fierce controversies. That is why I have selected them; and I have tried to depict them as glorious and profitable examples of Christianity, and of its persistent Unity in the midst of its most striking Variety.