Although admittedly limited, my experience with friends and students suggests that the average theist doesn't really understand the doctrine of creation ex nihilo. Rather, they accept the view that God created the universe from something like a reservoir of energy within him. As the chart above illustrates, this has serious implications. For if you're like them, then you are not a classical Anselmian theist. Rather, you're either a pantheist, a panentheist, or a demiurgist.
Now you might be fine with accepting that, but notice that you've also given up a lot in doing so. For example, you've given up most of natural theology. For now you accept that the stuff of the universe is eternal and uncreated. But perhaps the deepest problems lie ahead. For now you have a choice: you can say that (a) the materials are at least part of God, or you can say that (b) they are distinct from God. If you accept (a), then it's now hard to distinguish you from other pantheists and panentheists that are very far from classical theism. But if you accept (b), then you accept that matter is either a brute fact or a necessary being, in which case, again, kiss the cosmological argument goodbye. You can also say goodbye to the radical dependence of creation upon Creator, as well as a substantial amount of awe toward the transcendence, greatness, and otherness God.
On the other hand, you might want to sit tight with classical theism and hold on to the doctrine of creation ex nihilo. Your problem now is that the plausibility of the doctrine of creation ex nihilo is on an epistemic par with the hypothesis that the universe popped into existence out of nothing without any cause whatsoever.
So what's a classical theist to do? Probably the most sensible thing to do, if you are inclined to accept a religious conception of the world, is to reject classical theism and accept pantheism, panentheism, or (internalist or externalist) demiurgic theism. But then, again, it'll cost you a big cut in epistemic motivation and religious significance. Tough choices all around.
 If you accept the view of creation under discussion but you don't think the posited reservoir of energy within God is uncreated and eternal, then you think it either arose uncaused out of nothing, or you think God created the reservoir of energy within himself ex nihilo. The former view is incompatible with classical Anselmian theism; the latter reduces to the classical Anselmian theism, which is already accounted for on the chart. Read on for a critical problem with the latter's view of creation.