Alien Question for Atheists

[Angrboda]

If you mean they actually bring their own copy of the bible which is word for word the same as ours and worship a man named Jesus believed by them to have been born about 2,000 years ago, not on their planet, but in the region currently known as Israel on our planet .. well .. in that case I'd look around for the person who might be pranking me.

But if they merely believed in a God something like the Christian god, that would just lead me to believe their minds work something like our's, an instance of coevolution across worlds.

No, I'm thinking God would have taken form as one of their own species on their own planet, and had a different name, and didn't die on a cross. But the basis of the story would be the same: That this was considered "God's son" who came to show them the way and spread the same message. He performed miracles while he was there, started the new religion teaching them to love each other, and then was persecuted and sentenced to die a very horrible death for their sins. He soon rose from the dead and stayed with them for a while longer before heading off to Heaven. 

This religion has all the same fundamental principles as Christianity. Meaning they believe in the trinity (father, son, holy spirit), they do have a holy book much like the bible, consisting of writing before this Jesus character came, the story of while he was there which would be practically like the gospels, and further writing after he left. It would have very similar stories with the same principles (especially the story of their Jesus character, as I said above would be practically the same except on their planet) but wouldn't be "word for word" the same, as in like just a copy. And it wouldn't be called "bible" since they speak in a different language lol. They believe in salvation, believe in Heaven and Hell, and their views on morality revolve around love of God and love of neighbor, just like Christianity. They would have different denominations as well, but the basic fundamental tenets would be the exact same throughout, and would be the same as Christianity.

Imagine if that was the predominant belief system in their planet, how would you react?

Now that you start weakening the analogy, you also weaken it's persuasiveness. The more you scale back on the particulars, the more you create a space for things like convergent evolution. I have to ask, additionally, why you would take that step. Perhaps it's because even you don't have much interest in the answers to your hypothetical. Mahatma Ghandi has endorsed Christianity as a compatible belief, Krishnamurti I believe was even a Christian for a period, albeit a strange one [him or another swami, I forget who], and even the tantric yoga proponent Little Rik has said that he is also Christian. Just what exactly are the core elements of Christianity? You seem to be simply suggesting that the core elements of Christianity are what you believe them to be, regardless of whether that's justified or not. Proponents of a sort of universalism might well disagree and incorporate such "holy books" as the Aquarian Gospel, which, among other things, endorses reincarnation. This is beginning to suggest a similar flaw to your hypothetical as that which undercuts Pascal's wager. Your hypothetical starts looking less like a hypothetical than it is simply a disguised version of the question, "What if my God is true?" Something I find a great deal more mundane and plebian.