Argument One:
If from x’s point of view there is an objective fact about what time it presently is, then x is in time.
If x knows an objective fact about something, then from x’s point of view there is an objective fact about it.
If the A-theory of time is true, then there is an objective fact about what time it presently is.
God knows all objective facts.
So, if the A-theory of time is true, then God knows an objective fact about what time it presently is. (3 and 4)
So, if the A-theory of time is true, from God’s point of view there is an objective fact about time it presently is. (2 and 5)
So, if the A-theory of time is true, God is in time. (1 and 6)
Note that no claim is made that if the A-theory of time is true, God changes.
Argument Two:
God is actual.
Everything actual is in the actual world.
If the A-theory of time is true, the actual world is a temporally-centered world (one where there is a fact as to what time is present).
Anything that is in a temporally-centered world is in time.
So, if the A-theory of time is true, God is in time.
Many will dispute 3, but if we think of worlds as ways for everything to be, then I think it is hard to dispute 3.
I wonder if a classical theist who is an A-theorist might be able to respond that, yes, God is in time but God is not a temporal being. Compare that by doctrine of omnipresence, God is in space, but God is not a spatial being. Still, I think there is a difference. For as the above arguments show, the claim that God is in time is more limiting than the claim that God is spatially omnipresent—it is a claim that God is at the one objectively present point of time (he was and will be at others, of course).