"Christians" earn the term by relating to Jesus as described in the New Testament, believing he is "the Christ." Over two millennia, esp. since the Protestant schism, these groups have split up into various institutions called “denominations.” (Name = nom in = nomenclature) "Catholic" which once included most Christians is now one denomination among many others.
Church institutions are composed of “congregations” who are affiliated with their specific denomination. Some congregations are free-standing, organized by personal relationships or sometimes by declaring allegiance to the original Christian document, namely the New Testament, though they may put emphasis on different parts. These are not technically "religions" but denominations of the single religion, "Christianity." Different religions might be Judaism, Buddhism, Hindu, Islam.
Different denominational institutions within Christianity are, for instance, Lutheran, Baptist, Anglican, Quaker, Unitarian Universalist. Some are big and some are little. Most respond to the secular government, which gives them certain exemptions and protections because of their contribution to the quality of our lives. Their function is a bit like tribes and a few seem dubious, unworthy of recognition, at least to the government. Like snake-handlers. (I refuse to make a joke about professional courtesy.)