If a Jewish group sat down with a Christian conservative group, and there was a so-called Messianic Jew at the table, that would be the end of the meeting.
If you are trying to raise a child to be a Jew, then you have to create a sense of Jewish identity. You really weaken that sense of identity if you celebrate two religions.
We use American influence with Israel not to promote economic growth in the West Bank, but to try and impede Jewish - never Arab - construction in the capital city.