One of the big advances that St. Aquinas made was to insist, countering other popular philosophers of the thirteenth century, that there was only one Truth-not one truth for religion, another for science. Truth is one, even though our ways of groping for that truth are manifold. So the formulas we use-mathematical, philosophical, or whatever-can be very different, but they all are trying to get to the same place ultimately.
We have no data about other, nonhuman civilizations. They may not even exist; or they may be plentiful. (To insist that "God could not have made other worlds" was declared a heresy back in the thirteenth century-so this even covers alternate or parallel universes!) ETs may not be aware of the idea of an Incarnation, or they may have their own experience of the matter. Their experience may be so alien from ours that even though they have experienced God in their own way, it's an experience that we will never be able to share, nor they share in our experience.
I would suspect, though, that any conscious entity would wonder about the same things we wonder about-origins, meaning, etc.-and, just as we can learn from other cultures here on Earth, I would hope we could learn from other ET cultures.
Inevitably, any interaction with an unfamiliar culture results in some sort of evangelization. I recall my days in the Peace Corps, when kids in rural Africa would start adopting American customs, like wearing blue jeans, just because the Peace Corps teacher at their school wore blue jeans-even when the teacher tried not to influence the students!
So the question of whether or not one should evangelize is really a moot point. Any aliens we find will learn and change from contact with us, just as we will learn and change from contact with them. It's inevitable. And they'll be evangelizing us, too.
If we came across an ET culture that insisted 2+2=5, then we'd have to assume that either what they meant by 5 is what we meant by 4, or that one of us was seriously in error! Obviously we'd want to explore the matter further. "Evangelization" is what I would call this "exploration." We would clearly want to tell ETs what we have learned; we also want to listen to them, to hear what they have learned.
But one thing I know personally I have learned from studying eastern religions is that they have shown me the unique things that other cultures might not have had.for instance, the sense of a universe that is good, a creation of God, worth studying and worth caring for, and that one can come closer to God by immersing one's self in this universe (rather than trying to reject the physical world).
To withhold from an ET civilization a part of us as fundamental to ourselves as our religions-plural-would be dishonest, and certainly it would show no respect for them as equals. The important message that every Christian missionary has carried to each culture is that all people are equal and all of them are heirs to the knowledge of God that has been given to us. Soldiers might conquer them, secular philosophers might treat them as less than human (or, worse, condescend to them as "noble savages"), but the missionary can only accept them as equals.
Frankly, if you think about it, any creatures on any other planets, subject to the same laws of chemistry and physics as us, made of the same kinds of atoms, with an awareness and will recognizably like ours, would be at the very least our cousins in the cosmos. They would be so similar to us in all the essentials that I don't think you'd even have the right to call them aliens.