Apologia (from Greek ἀπολογία, "speaking in defense") is a formal defense of a position or action.   The term's current religious currency derives from John Henry Newman 's Apologia pro Vita Sua ( Latin : A defense of one's own life ),  which presented a formal defense of the history of his Christian life leading to his acceptance by the Catholic Church in 1845.  In modern usage, apologia describes a formal defense and should not be confused with the sense of the word 'apology' as the expression of regret. 
The etymology of apologia ( Greek : ἀπολογία ) is derived from the root word apologos (ἀπόλογος), “a speech in defense” . ,  and the corresponding verb form apologeisthai " to speak in one's defense."  The Greek philosophers Plato , Isocrates , and Aristotle described apologia as an oratory to defend positions or actions particularly in the sense of a legal defense.
Evolution of usage
The earliest English use of apologia followed from the Greek sense “a speech in defense". In 1590, a parallel meaning emerged meaning a "frank expression of regret". This parallel sense associated with "apologizing" for a wrong, progressively became the predominant usage until the 18th century when the older Latin meaning re-emerged to be recorded in 1784. This became the dominant meaning, owing in a large part to the publication of the influential work, Apologia pro Vita Sua , in 1865. 
Apologia Pro Vita Sua
John Henry Newman was regarded as a premiere religious figure even before writing his definitive essay, Apologia Pro Vita Sua . The backdrop for the essay was a heated mid-century theological controversy. Newman and other Anglicans were calling for the Anglican church to return to earlier a more disciplined traditions and an authoritarian hierarchy. Friction during the years from 1833 to 1841 led Newman and his allies in the Oxford Movement to publish a statement, the Tracts for the Times , to which Newman was a contributor. The tensions culminated in Newman's 1845 resignation as Anglican vicar of St. Mary's, Oxford and his departure from the Anglican church seeking to join the Roman Catholic Church.  . 
One of Newman's rivals was Anglican Charles Kingsley of the Broad Church party, who responded to Newman's departure with written attacks impeaching Newman's truthfulness and honor. Newman's response was the flowing, almost poetic prose of the Apologia Pro Vita Sua, offering a spiritual autobiographical defense to Kingsley's accusations. The book was ultimately very well received by Anglicans and Catholics and was influential in turning public opinion in favor of Newman. The book became a bestseller that remains in print today. Two years after its publication, Newman was ordained by the Roman Catholics and soon became established as one of the foremost exponents of Catholicism in England.  
In “The Evolution of the Rhetorical Genre of Apologia,” Sharon Downey argues that apologia has undergone significant changes because its function has changed throughout history. Downey takes on a critical generic approach to the feasibility of apologia. Halford Ryan advocates that kategoria and apologia need to be understood as a linked pair. Ryan proposes that a speech of kategoria or accusation motivates a defensive response, which should be treated academically as a rhetorical speech set.