Mmhhh... The eternal question of religion and science. That is a complicated debate as we enter the territory of the personal beliefs. And I will skip it and answer only from a scientific standpoint.

Here, I would say what is going on with God (or gods or anything that could play this role, depending on what one believes in) is completely disconnected from the topic: I am discussing some ways to understand the mechanism behind the functioning of the universe at the fundamental level: what is the nature of the building blocks of matter and how they interact.

By the way, this theory is not something I have proposed myself (other great physicists did). It is for now only theoretical: it may exist (and then we know how to look for it) or it may not exist. As it may exist, we need to be pragmatic and test it (to make sure not any interesting phenomenon is missed in data).

However, at the end of the day, we are discussing elementary particles and there is no real concern about the existence of mirror beings, mirror animals or even mirror gods. The abundance of the mirror chemical species being different from what they are in our world, I doubt mirror-us would exist (see my answer to @chappertron a bit further above in this post).