Probably most churches celebrate some version of the Lord's Supper, or Communion. Many of these churches offer this sacrament to anyone who walks in the door. Others, especially those that still believe the Bible, try to restrict this sacrament to those who profess to be Christians.
And then there are churches who offer the Lord's Supper only to those who are members of that church and are in good standing (not under church discipline) in that church. This practice is called closed communion.
The New American has an article about a Plymouth Brethren Church in England that is facing the loss of its tax exempt status because of its practice of closed communion (see British Gov't: Closed Communion Means Brethren Church Not a Charity).
There are good arguments for and against both open and closed communion. Personally, I lean toward closed communion. But the issue this article brings up is really more broad than who may or may not be admitted to the Lord's Supper. The issue is this: Can the state dictate what Christians believe?
If Churches ask the state for special treatment (tax exempt status), then they are placing themselves in a position where the state can indeed dictate what that church believes (open or closed communion, for example).
Please take some time to listen to Dr. David Murray's sermon:
Communion Table Address
Also Recommended reading/listening:
Pulpit Freedom Sunday - What's it All About? Part I
Pulpit Freedom Sunday - What's it All About? Part II
Pulpit Freedom Sunday - What's it All About? Part III
