If they go their way, Washington D.C. would be the only place where they are free to practice any religion.Yes.
It is illegal for Congress, not for any State.
Amendment I
Congress shall make no law respecting an establishment of religion, or prohibiting the free exercise thereof;
Moot. The States can do what they want with regard to religion within their own state lines. Each colony was founded on its own religious denomination, and the concern was simply to prevent the Federal government from imposing anything on any State.