Other than Creationists starting to wine around about that it can't be because it's not writen in the bible....
Well, it is not written in the bible that we will make contact with aliens, but it is written that we already had:
1st Book of Moses Chap 6:4
"The Nephilim where on Earth these days and the days after, as the sons of the true God had contuined realationsships with the daughters of mankind and their sons were born, they where the strong, from old ages, man of hounour."
It is also written in the Quran (Koran, holy book of the muslims):
Sura Al-Ma'arig (70) (The ladder to space):
70.2 ...it is from Allah, who has the ladder to space. 70.3 The angels and the angel Gabriel climb up to Him in ONE day, that last for 50.000 years 70.4 So stay patience 70.5 They say He is far away; 70.6 but we see He is near 70.7 at the day when the sky will be like molten metal... 70.10 they will be brought together in reach of their sight... (sounds to me like a discribtion of what EINSTEIN said about the relativness of time)
(english translation of me may be wrong, so read there for yourself)
If that is prooven to be right, it will sure end the leadership in the churches, because everybody now sees that they lied to us for 2000 years or more. I mean here: the leaders of churches lied not the holy books. (again: read for yourself)
And the impact on mankind depends on the kind of contact we make. If its only a radio message or something we discover with telescopes or so, than I think it would not have an big impact, because it like the news on TV. Noone has the real proof if it is true, faked or misused for propaganda.
The other thing would be a real physic contact...it is a difference of looking at a picture of a shark or tiger, or standing half a meter in front of those animals.
But maybe such sort of direct contact could help us to let loose our killer-instincs and shoot at someone else, than at ourselfes... (are you go to hell after killing an alien? Is it a diffence to killing a pig or chicken?)