Cult Is just the English version of the French word culte which means the public practice of worship, devotion, adoration, veneration or exaltation. A lieu de culte is any place of worship like a church, a mosque, a synagogue, or a Babylonian temple. Le culte protestant is Protestantism, le culte du soleil means “sun worshipers,” le culte de la performance means the veneration of productivity and competition, and a culte de la personnalité is the adoration of and obedience to a person - whether spiritual, political or entertainer.
This use of the word cult still makes the most sense in English, because we could say all of those French phrases in English and anyone would understand what they meant.
Therefore, by extension, all religions or sects that include devotion to a deity, person or idea are cults including Christianity as a whole and each of its denominations.