772–804 (conquests and revival); 794 (Council of Frankfurt); 809 (Council of Aachen on the filioque)

Charlemagne: conquests, the Carolingian Renaissance, and the Council of Frankfurt (794)

Charlemagne waged war against the Saxons for more than fifteen years, from 772. He forcibly baptized thousands and brutally suppressed resistance; the execution of 4,500 Saxon warriors is recorded. Saxony was subdued in 785. His 778 campaign into Muslim Spain ended with the destruction of his rearguard at Roncevaux, later celebrated in the "Song of Roland."

Charlemagne drew on invited scholars, chiefly Alcuin of York (at court 781–796). He also built on the earlier legacy of Bede the Venerable. He organized a network of cathedral and monastic schools and introduced a uniform Roman liturgy across the empire. Through Benedict of Aniane he reformed monasteries according to Benedict's Rule. This cultural upsurge is known as the "Carolingian Renaissance."

Theological disputes of the era included Adoptionism. It was the teaching of the Spanish bishops Elipandus of Toledo and Felix of Urgell. They held that Christ, in His humanity, was Son of God only "by adoption" rather than by nature. Alcuin accused them of splitting Christ into two sons. The error was condemned at the Council of Frankfurt in 794. The same council, due to a faulty Latin translation, also rejected the decisions of the Second Council of Nicaea on icons.

It also furthered the spread of the filioque formula, already introduced at Toledo in 589. Paulinus of Aquileia, at a local council at Friuli (796), gave what is apparently the earliest Western creed text with this addition. The filioque was officially confirmed at the Council of Aachen in 809. It became a lasting source of friction between East and West.