I am Christian which makes me Theistic, but I am not religious in the worldly sense. I believe God's existence is logically provable and I also believe the rules that he wants us to follow are also a priori, along with the fundamental reasons for these rules.

I think I sketch of God's existence would look something like Descartes Ontological Argument:

1. Our idea of God is an all perfect being. In other words, a being that has perfection in "all areas."
2. Existence is a perfection.
3. Therefore God must exist.

Common Criticisms of this sketch
1. Can't I assign the property of perfection to other things like unicorns and "deduce them into existence."
Rebuttal: Well we can't picture a perfect being with our mind, we can only make generalizations. For a spaghetti monster, we can picture what a spaghetti monster would look like making it already flawed. Now, let's not get this confused with Jesus who I believe came as God within a human body. The human body in of itself wasn't God, but the being using it was. Why can't God use a spaghetti monster or a unicorn? I mean I guess hypothetically God could do that, but those things (the spaghetti monster, unicorn, etc.) in of themselves wouldn't be God.
2. Isn't the idea of perfection kinda vague? Well not really. Descartes definition for perfection in of itself is a positive trait. For example, you wouldn't say weakness is a perfection, as weakness is merely the absence of power. A healthy food for thought would be to consider if we did consider weakness to be a perfection and figure out why this doesn't make sense.

For a more rigorous proof, I invite you to check out Godel's Ontological Argument if you know modal logic.

As for being Christian, I think this is at least a posteriori knowledge. There are many historical documents of Jesus's existence and his persecution. Along with his injuries, and sitings of him after he sustained these injuries (which would not be humanly possible based on the reported injuries).