Relatives of Colonel Count Claus Schenk von Stauffenberg fear Cruise will "inject" Scientology propaganda into the film depicting his failed assassination attempt on the Fhrer.
There were widespread reports in Hollywood last week that the actor is to play the leading role in the film, which is to be directed by Bryan Singer and made by United Artists, currently co-run by Cruise.
But the staunchly Catholic Stauffenberg family reacted in horror.
Count Caspar Schenk von Stauffenberg, the soldier's grandson, said: "I have nothing against him [Cruise] and can even separate his work from his beliefs in Scientology.
"But I and other family members are worried that the picture will be financed by the sect and be used to get across its propaganda.
"Unfortunately the family Stauffenberg can do nothing about this. My grandfather is a figure from history."
His grandfather was a decorated army officer attached to the German general staff, and lost an eye and one hand in combat.
He joined a band of conspirators in 1944 who vowed to topple Hitler because they saw he was leading Germany towards destruction.
Stauffenberg was invited to Hitler's military conferences at his headquarters in Rastenburg, East Prussia - now Poland - called the Wolf's Lair.
He planted a bomb in a briefing room next to the Fhrer on 20 July, 1944, that exploded and killed several high-ranking officers but Hitler survived because a massive oak table leg took the brunt of the explosion.
The bomb was intended to trigger Operation Valkyrie in Berlin, a military coup. But Hitler placed a phone call through to the military commander of the capital and the uprising was over before it began.
Stauffenberg was executed that same night and in the terrible vengeance that followed, 7,000 supporters and co-conspirators of the plot were executed, many of them strangled slowly with piano wire.
Their death throes were filmed for Hitler to watch in his private cinema.
Cruise, 44, is perhaps the most famous member of the Scientology cult that many governments and anti-sect movements say brainwashes its members. It was founded by science fiction writer L. Ron Hubbard in America in the 1950s.
In Germany, the sect is viewed with deep suspicion by the government and receives no charitable status. The government says it is a "fake religion" more interested in money than the spiritual well-being of its followers.
Cruise has lobbied on behalf of Scientology in Germany claiming the government denies its members "basic human rights".
The Stauffenberg film, which has the working title of the abortive coup, Operation Valkyrie, is seen by many as a way for Cruise to get back into the good books of both Hollywood moguls and his fans following a year in which his love life - and related strange behaviour - diminished his superstar quality.
Jumping on Oprah Winfrey's couch to declare his love for Katie Holmes did him no favours and he was later ditched by a studio following the box-office burnout of Mission Impossible: 3.
United Artists is now co-run by him and the epic is seen by Hollywood watchers as his attempts to relaunch his career.
But Stauffenberg is a complex character to play: an aristocrat, a patriot and a German.
"Three things that Cruise might have difficulty with, especially as accents are not his strong point," said one Internet critic.