It's darker than TFA, for sure... Lots of people die. Very little blood, though. As for Vader's "religion", two things: 1. The line likely refers to the Jedi, which are a distinct order (religion) from the Sith. Vader was a Jedi, Palpatine was only a Sith. Tarkin is one of the only people in the universe who figured out that Vader was actually Anakin Skywalker (it's explained in the new canon books and the Clone Wars series). Tarkin was one of the few people who met and worked with both Anakin Skywalker and Darth Vader extensively. It makes sense he pieced together the true identity of Vader. 2. Few knew the Emperor was Sith outside of Vader, Kenobi, and Yoda. He kept that fact hidden from the population both as Supreme Chancellor and as Emperor. As far as Tarkin knew, the Emperor was just a jacked up looking dude who really liked black. Palpatine never used his power publicly and even after the fight with Mace Windu, he didn't unveil he was a Force user.