But, being as far-sighted as he was ambitious, he resolved to secure for his enterprise the sanction of religious authority and the formal assent of the Estates of Normandy.
Not that he had any inclination to subordinate his power to that of the Pope.

Five years previously, Robert de Grandmesnil, abbot of St.Evroul, with whom William had got embroiled, had claimed to re-enter his monastery as master by virtue solely of an order from Pope Nicholas II.
"I will listen to the legates of the Pope, the common father of the faithful," said William, "if they come to me to speak of the Christian faith and religion; but if a monk of my Estates permit himself a single word beyond his place, I will have him hanged by his cowl from the highest oak of the nearest forest." When, in 1000, he denounced to Pope Alexander II.

the perjury of Harold, asking him at the same time to do him justice, he made no scruple about promising that, if the Pope authorized him to right himself by war, he would bring back the kingdom of England to obedience to the Holy See.

He had Lanfranc for his negotiator with the court of Rome, and Pope Alexander II.

had for chief counsellor the celebrated monk Hildebrand, who was destined to succeed him under the name of Gregory VII.