The Spanish word 'paganismo' (meaning 'paganism') traces back to the Latin word 'pagus', which meant 'rural district' or 'countryside'. From this root, the Latin term 'paganus' developed, originally meaning 'rural dweller'. During the early spread of Christianity, which was initially concentrated in urban areas, 'paganus' came to mean 'non-Christian' since many countryside dwellers maintained their traditional religious practices longer than city residents. This evolved into Late Latin 'paganismus', meaning 'paganism', which was then borrowed into Spanish as 'paganismo', combining the base 'pagano' ('pagan') with the suffix '-ismo' (indicating a doctrine or system of belief).

A simpler related Spanish word is 'pagano' (meaning 'pagan'), which is the base word from which 'paganismo' is formed. The suffix '-ismo' is a very common ending in Spanish that appears in many words describing beliefs, systems, or doctrines, such as 'cristianismo' (Christianity) or 'budismo' (Buddhism).

English speakers will recognize the cognate words 'pagan' and 'paganism', which share the exact same Latin ancestry as the Spanish terms. The English and Spanish words maintained very similar meanings throughout their development, both referring to non-Christian religious beliefs or practices. Another related English word is 'peasant', which, while now meaning 'poor farmer', also derives from Latin 'pagus' through Old French, maintaining the connection to rural life found in the original Latin term.