Ex nihilo nihil fit - from nothing comes nothing. This is a central axiom of many systems of thought. It underlies the law of conservation of matter, or of energy. This is the reason that Science formerly supported the idea of a solid-state universe so strongly, until it became untenable. It was why Aristotle supported some form of solid-state universe. Today, it underlies ideas like Zero-Energy hypotheses to try and explain existence.
So how does this fit Christianity and its idea of Creation? How did the Aristotlean Scholastics address this? Maybe we should address the idea of God.
For God is conceived as Being (as Philo put it) or I AM that I AM. He is seen as the ground of Being, the only thing that really has some existence of itself. This is why the Scholastics thought the phrase 'God does not exist' was illogical, as tantamount to saying existence doesn't exist. The only existence anything else has, is 'borrowed' of God.
The idea of a fishbowl universe with God looking down upon ours, is flawed and hopelessly anthropomorphic. This is not how it has classically been seen in Christian thought. God is both transcendant and immanent in the world. A weak Panentheism, such as Palamism, is perfectly Orthodox - in both senses of the word. While not fully pantheist usually, the action of God must be constant to maintain existence, which is thus in fact an ongoing act of Creation.
The problem is conceiving God as some form of material entity. Apophatic theology is explicit in denying this. The universe cannot be built from the body of God, like some primordial Bull or Gayomard or Frost Giant of old. God is other, not even really something that can be called 'substance'. The term 'ousia' though rendered substance, is more essence or ontological being. It is a metaphysical term.
Causality applies within the universe, of which God is substantially not a part. Our concept of causality can't really apply to God himself, as it is an applied term within our existence. In like vein, we cannot apply time or space either, that implies some form of materia. First we need to decide that causality applies to God, which the fairly unchanging traditional idea would not substantially support, then more importantly, what is matter for Him to act upon or come into being? We know how matter acts or changes, can describe it, but what is it substantially or in essence? Is it energy, or what? There are even ideas, such as Beverley's Idealism, that need to be addressed. The base ideas we are discussing needs to be brought into light for a discussion to progress.
We cannot conceive absolute nothingness. The very idea of absolute void, is beyond understanding. From this it follows that to conceive something by necessity requires the existence of something, be it a conceiver or even an Idea. Void is beyond us. God likewise is not really fully fathomable, hence the use of Apothatic thought or Revelation to know anything about Him. To apply the ideas from within creation to what created it,that what sets the rules must follow them, does not follow. The inconceivable doing the inconceivable is about as far as we are going to get, which is not too far off what the origins of existence are in an atheist perspective either, except for assumed agency in the former. For we all build what ideas we have on this event, depending we accept it, from what we see within existence and backtrack. There is no reason to affirm that what is a rule within is a rule without. I don't really see the positions that different, really.
Anyway, many Christian Theologians do not accept creation ex nihilo anyway, so this os a fairly moot point. It changes little, if anything, to the crux of Christianity (pun intended).
As an aside, you misunderstand Aristotle's four causes. They must all be more or less present, or one cause be assumed, for any to be operative. To artificially say only two apply, shows you are again talking about concepts you don't fully grasp. They are somewhat interdependant, a methodology to describe the Formal Cause largely.
This thread will likely be people talking past each other and misunderstanding one another, as we aren't really speaking from the same basis. It is sleight of hand.
@apogee is right, this has many semantic issues.
For Ex nihil nihil Fit! Now what did I mean by that?