What was Jacob Burckhardt’s first important historical book?

Jacob Burckhardt’s first important historical work was Die Zeit Konstantins des Grossen (1853; The Age of Constantine the Great, 1949).1

In this book, Burckhardt examined a transitional period of ancient civilization, characterizing it as a time that was both unhealthy and immoral, yet vibrant with cultural and religious activity. While he acknowledged the inevitability of the rise of Christianity, his historical perspective remained deeply sympathetic to the waning forces of the ancient world.1