You mean a fake extraterrestrial threat to attempt to unite humanity under one banner against one common enemy? They'll have an excuse to rewrite our understanding of religious texts, and create confusion about our beleifs of God. It will promote uniting under a one world government and one world currency. Hmm, how suspicious! It's almost like that's exactly what evil would want.

Also, a great excuse as to why a large mass of people dissapear.