But in spite of these characteristics of resemblance the two systems were fundamentally different. The Holy Roman Empire was consciously and deliberately a revival of a preexisting political institution that had been in existence before the birth of Christianity and was now revived under a specifically Christian character. Charlemagne assumed a title which had been held by heathen emperors before him, though the functions of his imperial office took upon themselves a specifically Christian character in consequence largely of his constant study of Saint Augustine's De Civitate Dei. But side by side with the Emperor was the Pope, and the Pope possessed spiritual authority and functions which were denied to the Emperor; as the Vicar of God upon earth, he ruled over and guided the souls of men, while it was the part of the Emperor to deal with the concerns of their bodies. As every student of the Holy Roman Empire knows, there was a long conflict over the problem of the true relationship between these two independent authorities; but throughout the centuries during which the Holy Roman Empire was a living force in Europe, the distinction between tem poral and spiritual authority was never lost sight of.