Saying a believer in spiritual beings has to believe in God too is like saying a believer in intelligent life on other planets has to believe in Darth Vader too. God is a character of literary fiction and nothing more.
It’s more of saying that gods and spiritual beings are varying degrees of the same thing. Gods and spiritual things can come in any manner of unjustifiable configurations, and they are all products of imagination. Belief in the possibility of alien life or gods is one thing, believing in their actual existence is another.
I'd say that it's possible to believe in the supernatural (in some sense) without personalizing it into a god and turning it into an object of religious worship. Hence without it becoming theism.
It’s the belief in the actuality of supernatural entities that defines theism, not necessarily the worship of them. I could conceptualize a god that exclusively rules over dog shit, that has no obvious merits for human worship, yet a belief in such a deity would qualify as theism.
Nope. There are no rules of make-believe. One can certainly think there are ghosts or spirits and no God.
The lack of rules regarding the conception of supernatural entities is what defines all such entities as being cut from the same unsubstantiated cloth. When you believe in imaginary supernatural beings, you believe in deities. When you believe in deities you engage in theism.
People do not need Gods to believe in ghosts, tree spirits, ancestor spirits, reincarnation, and so forth.
A god is just one example of deification, ghosts, tree spirits, ancestor spirits, reincarnation, and so forth are others.
You have that reality directly in front of you - it's not a theoretical matter.
The reality is that these groups believe in deities that are not directly in front of anything.
Theists do it by postulating deities. Atheists do it without postulating deities.
Postulating deities doesn’t qualify one as a theist, it’s believing in the actuality of that postulation that does.