There are two interpretations . Traditional Catholics consider non-Catholics as protestants. Historians consider Christian groups that historicaly protested or antagonized the Catholic Church as protestant, these are typically Lutherans and affiliates. Are the Greek ,Russian and Coptic Orthodox Churches protestant?They have only been geographically separated and developed that way . They maintain many sacraments seen in the Catholic Church. Also Christian coalitions have core requirements like acceptance of the Divinity of Jesus as the Son of God, Holy Trinity, Ressurection and Salvation through grace, personal relation to J.C. etc. Jehovah's Witnesses, Adventists, etc, don't accept all of these. Even the Catholic Church accepts (I believe) certain groups as Christian even if they don't recognize the Pope as long as the fulfill these Christian requirements. Since this is an encyclopedia, we need to distinguish these forms.