Gee, I wonder how many theists get questions like this? Well, fortunately the question doesn’t depend on any theistic frame, but only on the science.
Recalling that roughly 45 percent of the American people are Young Earth creationists who don’t accept that there was a Big Bang, here’s the go for the rest who do pay attention to the science.
In modern Einsteinian relativity physics, time & space are not separate, but rather features of a unified timespace. What we call “time” is nature’s way of not having everything happen all at once. The Big Bang concerns the origin of that spacetime stuff, initially essentially a “singularity” of the sort that goes on inside black holes. In a sense, the universe is an expanding “white” hole spewed out from an initial singularity (cause unknown, fill in blank to suit).
To speak of time “before” that is off the scope, just as asking how does move up or down before there is a space to do that in. Now whether there is (or was) some meta spacetime in which multiverses or some such exist for their own “time in that frame is anybody’s guess (Brane theory relates to such things, Cheshire Cat smile optional). Nothing in Big Bang cosmology says anything about (or needs anything) “before” or “apart from” the Big Bang itself, though, even if there may have been (or still are) “time” going on outside the time we’re used to inside of this universe.
Remember that what we call time is a slippery thing, in which what is seen as before or after something else can depend on how fast you’re moving. To what extent there is some absolute TIME in which all the events can be laid out independent of the motion frame is a fun opportunity to get physicists to fist fight, or whether or not even gods can sort that out, is a matter of (as yet) unverifiable philosophy.
It would obviously have helped some for the god hypothesis for any of the known texts recounting such things to have taken note of that timespace stuff, and provided some skivvy on it. But alas we must depend on the trusty old science method to work these things out, provided we get the time to do it. Tick tick tick.