Complex sentence: Gratefully did the Roman population acknowledge that they had escaped subjection to the Lombards only through the mediation of the pope, for it was only for the pope's sake that Pepin had resolved to interfere.
Split into simpler sentences: Gratefully did the Roman population acknowledge that they had escaped subjection to the Lombards only through the mediation of the pope. For it was only for the pope's sake that Pepin had resolved to interfere.