One common objection (e.g., Grünbaum) to your view of the universe's beginning is that the moment of creation cannot be "before" the universe's actualization, since that already presupposes the time of the universe. In response, you've proposed that perhaps the moment of creation of the universe was simultaneous with the universe's beginning, thus no longer needing a "before".
But our notion of what it is for an event to be simultaneous with another event can only make sense within an already existing space and time (irrespective of whether simultaneity is taken here as absolute or relative).
So to talk of space and time itself as being in a simultaneous relationship with a cause or moment of creation (as if space and time is a spatio-temporal thing or event itself) seems unintelligible. Our notion of simultaneity, or coincidence, and even our notion of what it is for something to be an event, surely can only make sense within an already existing space and time.
Lastly, if one claims that perhaps the property of simultaneity itself also begins at the SAME TIME (or "simultaneous") with the universe's beginning and its cause, then one would seem require a second-order simultaneity, which again seems unintelligible.
That's a pretty solid counter to Craig's past arguments. In his response, Craig gets into more philosophical ideas of what time actually is:
The key assumption underlying your objection presupposes the truth of the substantival view of time. For it assumes that time is explanatorily prior to the occurrence of events. It assumes that in order for events to occur, time must, so to speak, already be there. But on a relational view that is false. Time exists because the events occur. The happening of events is explanatorily prior to the existence of time
Basically, the "substantival" view of time says that time exists even if events don't actually occur; the "relational" view says that time only exists when events occur. Craig concludes,
So on a relational view of time, God existing changelessly sans creation would be timeless. As Leibniz rightly saw, time comes into existence with the occurrence of the first event, God’s act of creation. Time begins to exist because an event occurs.
So your objection must presuppose the untenability of a relational view of time. But such a view seems perfectly coherent and is widely held today. So the objection is based on an assumption which the theist is free to reject.
There's a very thorough explanation of these theories here, but I want to cut to the chase. We've never actually observed time existing without any events (obviously), but we also have no particular reason to believe that the existence of time is contingent on events. A good summary from the linked article from the IEP:
For the relational theory, all that exists are physical objects and their relationships, so its temporal continuum is just a useful fiction created by mathematical abstraction from what really exists. Since this continuum is not just a fiction, we should reject the relational theory.I'd be inclined to say that a relational theory is less coherent than a substantival one, but I'm fine with saying that I don't know which is correct. Maybe they're both wrong, or both right given certain contexts and interpretations.
But what I notice here is that Craig shifts the burden of proof. Of course, Craig cannot establish which view of time is correct; so regardless of which view his theology required, he could respond identically. So he says that the theist is "free to reject" the idea that the relational view of time is untenable. But he's putting the cart before the horse – if you already assume that God exists, and you assume God exists in some timeless, changeless form independently of the physical universe, then you can appeal to the relational view of time to justify your position.
But how do you get to that position in the first place? What evidence is there that a timeless, changeless God exists independently of the universe? I've argued extensively in the past that such a conclusion cannot be arrived upon without making a great deal of question-begging assumptions, and Craig's conclusion here seems no different. An atheistic view does not require us to presuppose any particular view of time; we can remain "time-view-agnostic". But Craig's theology requires him to assume the validity of the relational view, and the best he's got is nothing more than 'you can't disprove it'. That's clearly question-begging.
So Craig assumes the relational view of time to justify his adherence to his theology, and uses his theology to justify his adherence to the relational view. Checkmate, atheists!