Pope Saint Leo III
The Church in the Dark Ages, pages 399-400: It was then that an episode of capital importance occurred, which puts in its true light the much more famous fact of the coronation which was soon to follow. In what conditions could the Pope reestablish himself in the Papal See? Since he had been the victim of an odious plot, it ought to have been enough to restore his powers to him and to punish the criminals who had made the attempt upon his life. But this was not what happened. Leo III re-entered the Holy City in the autumn of 799, to be followed, a year later, by Charlemagne himself. The Frankish king was welcomed by the Pope and by a gigantic procession when he was 7 miles from Rome, and was received in great pomp on the steps of St. Peter’s on 24th November. He seemed to be celebrating his triumph. What was he going to do in Rome? One chronicler tells us quite unequivocally: ‘He was going to carry out an investigation into the crime of which the pontiff was accused.’ In other words, the temporal sovereign was setting himself up, if not as judge, at least as the appraiser of the conduct of the Sovereign Pontiff. Nothing better shows how dependent on Frankish power the successor of the proud Hadrian I had now become! It was true, of course that no one really dared judge the Pope, and this process was undertaken with the declared intention of clearing his name; Alcuin had reminded his master of the famous maxim which went back to the difference between Pope Symmachus and Laurence, the antipope: ‘No one can sit in judgment upon the Holy See.’
The Catholic Encyclopedia in its entry on Pope St. Leo III by Horace K. Mann describes what happened next: In the following year (800) Charlemagne himself came to Rome, and the pope and his accusers were brought face to face. The assembled bishops declared that they had no right to judge the pope; but Leo of his own free will, in order, as he said, to dissipate any suspicions in men’s minds, declared on oath that he was wholly guiltless of the charges which had been brought against him. At his special request the death sentence which had been passed upon his principal enemies was commuted into a sentence of exile.