'apostolicness' originates from Late Latin and Greek elements: from Late Latin 'apostolicus' and Greek 'apostolikos', ultimately from 'apostolos' meaning 'one sent away' or 'messenger'. The suffix '-ness' comes from Old English '-nǣs' forming abstract nouns.

'apostolicness' developed by combining the adjective 'apostolic' (from Late Latin 'apostolicus' < Greek 'apostolikos' < Greek 'apostolos') with the English nominalizing suffix '-ness'; 'apostolic' entered English via Church Latin and Middle English usage and eventually produced the abstract noun 'apostolicness'.

Initially related simply to being 'of or pertaining to an apostle' or 'sent one'; with the addition of '-ness' it came to mean the abstract quality or condition of being apostolic, a usage that continues in modern theological and ecclesiastical contexts.