Timeline of Roman Catholic Innovations

[Barbara]

Thank you, Father Joseph.

Was there a particular reason why unleavened bread was starting to be officially mandated besides the contact with Armenians ?
Broader contacts with the Greek Orthodox, perhaps ?

[haralampopoulosjc]

No, Western Orthodox used leavened bread for the most part, but a few places in the West started following the Armenian heretics and using unleavened bread. By the time of the schism, leavened bread in the West was still a majority use but Popes had begun to mandate unleavened bread.

Is there a source on the connection between the Armenians and the introduction of unleavened bread in the West?