Now, back when they made that parallel dimension, they also made the Testament, in order to repeat the steps that got them to ascend in the first place. Their theory is to repeat the effect by repeating the cause. Scientifically sound idea, right?
Well, it doesn't quite work as well as they think. The history the Testament provides is often vague, inaccurate, and only given in chunks of a century or 2 at a time. All this fear of an "apocalypse" is due to the Testament not updating when it's supposed to. They're running out of things to recreate, and the magic book is not coughing up the next script.
There's also self-interest mucking things up, like people who take on historic namesakes not wanting to die when they're supposed to. This is actually a major plot point of S2.
So no, they're not in the past, they're tens of thousands of years in the future, and they're always in that future. They're history reenactors, like the guys who go out on the weekend in Civil War uniforms, except there's actually a point to it this time.
There's a wikia you can google up, if you have questions about more specific things.