hierarchy: [14] Greek hierós meant ‘sacred, holy’. Combined with -arkhēs ‘ruling’ (as in English archbishop) it produced hierárkhēs ‘chief priest’. A derivative of this, hierarkhíā, passed via medieval Latin hierarchia and Old French ierarchie into Middle English as ierarchie (the modern spelling was introduced on the basis of the Latin form in the 16th century).
At first the word was used in English for the medieval categorization of angels (into cherubs and seraphs, powers and dominions, etc), and it was not until the early 17th century that it was applied to the clergy and their grades and ranks. The metaphorical use for any graded system soon followed. [hierarchy etymology, hierarchy origin, 英语词源]