The True Way was a Cardassian separatist, extremist, terrorist organization that operated in the 2370s. They believed the Federation to be the greatest threat to the Cardassian people, and the cause of the Cardassian Union's economic and political difficulties. Until 2372 they had never committed any acts of terrorism against the Federation or Bajor.
Regarding what became of the True Way after Cardassia allied itself with the Dominion in "By Inferno's Light", Ronald D. Moore commented: "I think this group was made irrelevant when the Cardassians joined the Dominion. I think our initial concepts were that they were a sort of fundamentalist group dedicated to the return of the old Cardassian government." (AOL chat, 1997)
The True Way reemerged in 2376, where they tried to destabilize the new Cardassian government. Elim Garak revealed that he once joined the group to spy on them. (Pocket DS9: Worlds of Star Trek: Deep Space Nine, Volume One)
The True Way featured in Star Trek: The Fall as the assassins of the current Federation President, although this revelation is concealed to both protect Cardassia's new alliance with the Federation and as part of a covert attempt by parties in the Federation to encourage Starfleet to maintain a more pro-active, offensive approach by framing members of the newly-formed Typhon Pact for the assault. These efforts are thwarted when Garak is elected to the new position of Cardassian leader while Captain Jean-Luc Picard, Fleet Admiral Leonard James Akaar, and newly-promoted Admiral William T. Riker expose the Federation internal conspiracy, although they avoid identifying the true assassin to maintain peace.
The True Way emerged once more in 2383 to denounce the resurgence of the Oralians, a religious group that dated back to the Hebitian civilization and was previously outlawed. By 2409, they had become deeply rooted in Cardassian space, waging a campaign of terror and fear throughout the region, even seeking an alliance with renegade Dominion forces led by Laas and Lamat'Ukan in an attempt to expel the Federation, and return Cardassia to its old ways. (Star Trek Online)