I thought the Amharic and the Tigrinya were the two ethic groups that traditionally made up the indigenous church.
Both the Amhara and the Tigrinya are the most Westernized peoples of Ethiopia (I have a few CDs of jazz fusion records made by the hip soulsters of swinging 60s Addis Ababa) and were therefore the segment of the population most likely to be introduced to Marxism by elite members who went to Oxford, the Sorbonne and Turin for their education.
Think of them like the Italians they despise (and resemble) - some are extremely pious Christians and some are hardcore Red Brigade types.