And to clear up the Catholic communion thing, some churches practice "open" communion and some practice "closed" communion, which means they only want members of the faith participating. That is because they see it as having a specific meaning and that if you do not share that significance then you should not participate. Catholic communion is very much closed because their beliefs are among the most extreme: that once the rite is performed then the bread and wine become the flesh and blood of Jesus, which takes Jesus's words literally while most other Christians believe they were meant figuratively.
As you can imagine, this is about as sensitive as it gets with Catholics, who don't want to see people consuming their savior without a sincere belief in what they are doing.