For sheer drama, very few of the notable set pieces in the long history of the papacy have matched that of February 1240. With the formidable Emperor Frederick II and his army having first entered the valley of Spoleto, then taken the cities of Viterbo and Montefiascone, as well as various castles of the Church, and now threatening the invasion of Rome itself, backed by the increasingly vociferous support of the ever-fickle Romans (‘Let the emperor come. Let him come and receive the City!’), Gregory IX, in extremis, his cause apparently lost, played the final desperate weapon in his depleted armoury:

Taking the crown from his own head he placed it upon the skull of St Peter and then upon the skull of St Paul and called out:

Then, taking with him not only the skulls of the saints but the relic of the True Cross, moving in solemn procession to the church of Saint Peter, Gregory there preached to the people:

Offering them the general indulgence of the Apostolic See, Gregory thus successfully rallied the Romans to his cause, with many of them taking up the sign of the Cross in defence of the Church, while Frederick, realizing he could achieve nothing more, withdrew to Apulia.

This is Gregory IX – masterful, energetic, courageous, unyielding. This is Gregory IX – confrontational, obsessed, ‘a hate-filled stubborn old man’, as Kantorowicz delicately described him.

Nobody (except the man himself) has ever questioned the zeal of Gregory's faith. Even though ‘the Inquisition’ had a long pre-history, certainly stretching back to Lucius III's Ad abolendam, the formal beginnings of the institution are most associated with Gregory. It was he who issued the bull Excommunicamus of February 1231 and there provided the penalties, including the ultimate punishment, for the various groups defined as heretics – Cathars, Patarenes, the Poor of Lyons and many more (all of them tied together by their tails) – as well as their defenders and supporters.