I have always wondered how an ETI would perceive the Christian
symbol of a man nailed to a cross, assuming they would be able to
recognize the situation at all.

Would we be the height of barbarism to them, that we "worship" a
person being tortured to death, in essence? And one who is supposed
to have been sent by the Ultimate Being of the Cosmos to "save" us,
no less. They might be surprised we weren't vaporized on the spot for
doing what we did.

I keep thinking of the opening narrative of the novel version of Douglas
Adams The Hitch-Hiker's Guide to the Galaxy, where humans are
explained as the type of creatures who once nailed a guy to a tree
for saying let's all be nice to each other.

In the famous Pearl S. Buck novel, The Good Earth, the main character
(a native of China from over a century ago) who comes across an image
of Christ on the Cross without knowing the Christianity behind it,
assumes that Jesus must have been a very bad criminal to deserve
such a punishment and leaves the subject at that (as far as I can
recall from high school). I don't mean to make Chinese culture sound
or seem "alien", but the point I am trying to make is that if another
human albeit non-Western society does not get what is going on with
the Christian Cross, just imagine what an alien might make of it - or not.

It will be interesting to see if any ETI have religion, or if it is a quirk
unique to our little and still developing brains.