Protestant theology claims that Scripture is self-convincing. Its autopistia however easily becomes an abstract confessional doctrine unrelated to exegesis and biblical theology. Rome and Reformation have discussed the question whether the authority - or at least the authorization - of Scripture is a matter of the authority of the church or ultimately rests in the witness of the Holy Spirit, as John Calvin claims. Seldom however has this issue been related to the original acceptance of a canonical book. Indeed, the acceptance of the New Testament books by early Christians as authoritative Scripture was a complex process rooted in their alignment with the Old Testament, their apostolic authority, and their transformative power. This paper explores the content of the epistles of Paul to the Corinthians to answer to the question why - given the initial rejection of Paul's apostolic authority and the tense relationship between the apostle - these letters were ultimately accepted as authoritative apostolic witnesses. By highlighting the internal criteria of its 'canonicity' this paper seeks to shed light on the autopistia of Scripture in actu.