Father Theodore was a Catholic priest and part of a Roman delegation sent by Pope Constantine in AD 769, to a heathen Bulgar Khan named Telerikh. The purpose was to attempt to convert him and his people to Christianity. The other two emissaries were Father Niketas and a monk named Paul. Theodore was a quick tempered, doctrinaire man but very knowledgeable in theology.
The evening after their arrival in the Bulgar capital of Pliska, a delegation of Muslims also arrived intending to try to convert Telerikh to Islam. Telerikh had invited both delegations in order for them to present their respective faiths and debate each other. He would then decide which, if any, he and his people would convert to.
At the first meeting, both delegations presented gifts to Telerikh, including a Bible and a Qu'ran. This led to a theological discussions between Paul and Jalal ad-Din on the differences between their religions. Theodore made several angry responses to ad-Din on things he had said that Theodore found blasphemous. These were unhelpful and Paul restrained him from angrily arguing with ad-Din.
After considerable time, the Kahn stated that these issues needed thought and dismissed the parties until another meeting three days later. He also warned the Christians and Muslims that they all were his guests and that they were not to fight one another.