Every serious Russian religious text before Avvakum was written in Church Slavonic, the formal literary and liturgical language inherited from Orthodoxy's Byzantine roots. Avvakum broke from that convention almost entirely, writing his own life story in the colloquial Russian people actually spoke, and literary historians treat that choice as a genuine turning point, the first significant Russian book to sound like a real voice rather than a formal register.
The content matched the style. Avvakum used the autobiography as a direct, first person argument for the Old Believer position, recounting his own exiles, imprisonments and sufferings as evidence that the older rites, the ones Patriarch Nikon's reforms had overturned, were worth that cost. For Old Believer communities that would spend the following centuries as a persecuted minority within Russian Orthodoxy, Avvakum's own account of enduring persecution for the old ways became a founding text of communal identity and something close to hagiography, a saint's life told in the saint's own unpolished words rather than composed after the fact by someone else. Its vernacular style and its religious argument were never separable; the plain language was itself part of the case being made.