Why couldn't this be the truth? I will not claim to be a Christian but can see relevance in much of what Soldier has said (although some of what has been said may be overstated).
There is no way of knowing what God is or what Angels / Demons are. Some of the biblical stories recounting experiences with these entities could very well be based upon interactions with extra terrestrials, while others I would argue, are not.
Heedless, why not accept the 'christian' prophesies of an end time? While I wouldn't say that extra terrestrials are 'fallen angels,' I would definitely be willing to accept that they have the means to fulfill the acts portrayed in the bible.
On top of that, look at where technology has taken the human race. We haven't yet developed a means to cure cancer (that we know of at least) and already we are on the brink of WW3 - complete destruction. One argument that cold be made in lieu of this evidence is that as technology advances, the human race becomes further distant from god.
Why can't we then assume that extra terrestrial visitors - who are much more advanced than we are - are further from god? Even further perverted by the 'godlike' powers they have on Earth?
I would like to state that in spite of all this I believe that humanity is completely capable of fulfilling the end time prophesies on their own. The Maya prophesied their doom by the hands of evil gods and low and behold in came the Spanish. Whoever or whatever extra terrestrials may be, the biggest threat to humanity is itself.