He may have been a Jew but he did not have Jewish beliefs.(And I suspect this was not an Orthodox Jewish bar mitzvah, if nobody reacted to such a statement.) Judaism is a religion and it has beliefs associated with it. One of those beliefs is that a Messiah is coming. For purposes of this discussion, we can substitute the word “Judaism” for “Jew” and my point remains.
Judaism awaits a Messiah that Christianity teaches has already come. The religions are opposed on this point. Messianic expectation is a key idea involved in the Seder. This makes it an inappropriate for Christians.
No, he absolutely wasn’t Orthodox, but I don’t think you understand me.
a) Judaism doesn’t have a Vatican that excommunicates you for saying things that a teaching authority deems to be untrue. Even to the Orthodox, if your mom is a Jew, you’re a Jew. You can be a ignorant Jew, you can be a bad Jew, you can be a non-practicing Jew, you can be a Jew whose name isn’t going to make it into the Book of Life, but you’re still a Jew. This is particularly true since the Shoah…Hitler did not care if you passed a litmus test of belief or even of performance. There is a great deal of solidarity among Jews that comes from knowing that all the same people hate them and would like to see them all utterly wiped from the face of the earth. This is not to say that there aren’t Jews who consider those who practice Christianity to be dead because of their apostasy. Not every Jewish congregation thinks that way, though.
b) Judaism, even from a religious standpoint, is far more concerned with what you do than with what you think. Yes, you will find Jews who believe that once you’ve gotten to atheism, you’ve abandoned Judaism, but even those would be mostly concerned that you don’t perform the mitzvahs required of a Jew, not that you don’t have the right ideas in your head.
At any rate, if you were to explain your point of view to a Jew who doesn’t believe a literal messiah is coming with an “oh, well, your seder is different, it’s hardly a seder, because what you believe isn’t actually Judaism”, be sure he’d beg to differ. Unless this Jew was a very gracious friend, your statement would at least solve your problem of how to handle his invitation to come to a seder at his house!
You will even find Jews who become Christians and still consider themselves Jewish. Some of these organize under the name “Jews for Jesus”:
jewsforjesus.org/