"To assert it's ethnically based is nonsense."
To assert it is purely ethnically based IS nonsense.
But there is certainly an ethnic compenent --- to the Jews, Jews are, by definition, descendants of Abraham, Issac, et al.
To reject that is nonsense, as well.
I am certainly an ethnic Jew, my acceptance of Christ or no.
You miss the argument as you are buttressing my point.
You may be an "ethnic" Jews- whatever that is- but you are no longer accepted as part of the Jewish people. You would not be counted for a minyan- for example.
On the other hand, someone not an "ethnic" like you, would be fully counted if they converted.
Ethnicity is not the right word. It is a TRIBE!! If you choose to leave the tribe (like through conversion out) you may have had Jewish parents, but you are NOT a member of the tribe. You can have a wonderful family and be a good Christian, but you are not longer a member of the tribe. I joined the tribe. I did not change my ethnicity, neither did the children who were adopted from Cambodia, or who were born to black parents. They also joined the tribe.
A third of the children in my son's Jewish private school has one parent who is a convert and joined the tribe. We have an Irish Jew who married an American of Irish descent who converted. It is tribal identity, not ethnicity. Looking at it this way incorporates born Jews who choose not to observe their religion.