I don´t think that this has been established.
Without going into the scientific proofs (big bang) suffice me to say that if the universe was eternal it would be outside of time. However, time is the measurement of change, hence, to be outside of time is to be outside of change. We know that the universe is changing, so it must be in time (not eternal).
The only other alternative is to say that the universe is infinite, namely without a beginning. But what exists must have a cause. The series of causes cannot be infinite, for that would not account for the effect, namely, the universe. There must be a cause that does not need a cause. However, this cause must be uncaused, and unchanging, that is, outside, or not subject to time (eternal). This is the uncaused cause that created the known universe. Being must be grounded in something that does not need to be grounded in something else. The universe cannot be this uncaused cause because it is changing and the ultimate cause must be unchanging, namely, have no potency for change (or anything). This is because it is in/subject to time, and time cannot be infinite. If time had no beginning we would not reach the present moment. In other words, as Aristotle and Aquinas put it, this being must be pure act, pure existence.
Doens´t change anything about the fact that creation process must be a temporal one (i.e. happen within time), if there is a state of the universe not existing and a state of the universe existing (and both are not eternal themselves). Whether it was "decreed from all eternity" (whatever that might mean) or not, doesn´t help to solve this dilemma.
There is no contradiction with an eternal being decreeing to create something that needs a beginning. In fact it is necessary for the above reasons.
there is no such thing as a state of the universe not existing. "not existing" is not a state, it is nothing.
The contradiction is there, as soon as you ascribe temporal actions to this being outside of time.
I did not ascribe temporal actions to this being that is outside of time. The fact that he created time does not make him subject to it. There was no "before" he created. It is a category mistake to say "what existed before time?" nothing temporal existed without the universe.
Happy Easter to you too, and thanks for the dialogue!