Famous atheist Michael Shermer asks that question here. It’s a good overview of things to think about in this regard. I am in basic agreement that the existence of an advanced ET would be embraced as God by many, perhaps most. But despite the article’s strengths, it misses obvious “tests” — like prove to us, Mr. ET, that you (a) can create matter itself, from nothing; and (b) that you’re the entity who did so aeons ago.  The “aeons” part of that also would require the ET to prove that his species no longer dies (or at least is not capable of death). I list this as an omission since this is part and parcel of any theistic religion’s definition of God. Third, there is the matter of not being part of the material creation. Any theistic religion believes God created all matter – all that is, “visible and invisible” to borrow the New Testament language. Without being able to demonstrate this attribute, ET would not be accepted as God.

Anyway, I could pick at this more, but you get the idea.