Are Christians allowed inside a Catholic Church?

Answer 1

Everyone is welcome inside a Catholic Church. Non-catholics, however, should not receieve communion, as beliefs are different.

Roman Catholic AnswerYes, everyone is welcome inside a Catholic Church, but your question doesn't make sense as Catholics are Christians.

from A Catholic Dictionary, edited by Donald Attwater, Second edition, revised 1957

Christian

A name first given to the followers of our Lord at Antioch (Acts xi, 26). Since the rise of Protestantism the name has been used in so many different senses as to have become almost meaningless: it may indicate a Catholic or a Unitarian, or even be applied to an infidel who displays some virtue which is associated with Christ. It may reasonably be applied to the members of all the ancient churches, whether in communion with the Holy See, or not, and to those Protestants who profess, explicitly or implicitly, the Nicaean creed in its traditional interpretation. The Church puts no definite official meaning on the word, as she does on Catholic.