Religion: Sword of Judgement (leader)
Alter Ego: Andrei Rostov
Other Names: Piston; Andrei Semyanovitch Rostov; Andre Rostov
First Appearance (Additional Details): (as Agamemnon) Marvel Two-in-One (vol. 1) #10 (July 1975): "Is This the Day the World Ends?"; (as Piston) The Uncanny X-Men (vol. 1) #261 (May 1990): "Harriers Hunt"
Number of Appearances: 2
Enemy of: Black Widow
Ally: The Thing
Ally: Natasha Romanoff (Black Widow)
Note: Black Widow's former classmate, boyfriend
Agamemnon led a rather large terrorist organization known as "The Sword of Judgement." It was well-funded, multi-national and multi-cultural. Its aims seem a bit vague, however, or perhaps simply broad. In Agamemnon's own words, they are "revolutionaries, the avenging sons of all the exploited peoples of the world, gathered to wreak bloody vengeance on our oppressors."
We are going to assume that Andrei Rostov, the Russian revolutionary who appeared in Marvel Two-In-One #10 (1974), is the same person as the equally non-superpowered armor-wearing mercenary named Andrei Rostov who appeared in Uncanny X-Men #261 (May 1990). Both characters have the same name. Andrei Rostov was captured and presumably sent to jail at the end of Marvel Two-In-One #10. Presumably he escaped or was released in at some point and crafted a new identity as "Piston."
Both stories were written by the same author: Chris Claremont. Presumably he knew what he was doing and intentionally brought this character back.
Suggested links for further research about this character and the character's religious affiliation: