Theists also embrace the notion that non-existence has a causal relationship with existence: at t=0, God instantly exists for no reason. If something as complex as a God can instantly exist for no reason, anything can.
In fact, almost anything else would be more likely. If there was ever a state of absolutely nothing, then by definition no laws governed what would happen to it, therefore with no constraints on what can happen, literally anything can happen. And with no laws constraining which thing will happen, every possible thing that can happen is equally likely to happen.
From this it follows that a vast multiverse is the inevitable outcome of any absolute nothing-state. Because for a nothing-state to remain nothing is just one possible thing that can happen, out of infinitely many things, and each thing is equally likely, therefore the odds that a nothing-state, constrained by no laws, will remain nothing is infinity to one against.
What will instead most probably come to exist (with a probability infinitesimally close to 100%) is a vast number of universes. Because the number of universes (including zero as one number; one universe as another number; two universes as another; and so on) selected at random from among all possible numbers of universes will be a virtual infinity (any finite number or fewer universes will have odds of existing of infinity to one against, while the odds that a larger number will exist is the converse probability, which is infinity to one in favor).
All of this follows necessarily from positing that nothing exists, not even any laws that would constrain what can then happen to that nothing.
For the whole demonstration see Ex Nihilo Onus Merdae Fit.