IF Christianity is a subset of Religion and IF Religion is
  defined ONLY by the subset: (the set being: "a pursuit or
  interest to which someone ascribes supreme importance." and the
  subset being "the belief in and worship of a superhuman
  controlling power, especially a personal God or gods." ) AND IF:
  belief is a required property of religion and therefore its
  subset, Christianity AND IF Atheism is defined as "disbelief or
  lack of belief in the existence of God or gods" AND IF belief in
  the existence of God or gods is a requirement of at LEAST
  Christianity (subset) and possbly the whole set (Religion) THEN
  (perhaps) there could be something to debate about. That is, if
  those were definitions I considered to have exclusive validity.