Three lines of argument converge to persuade us of the sufficiency of Scripture:-
(a) Jesus Christ. As Heb 1:1-2 asserts, the previous, and partial revelation of God is completed by the final and perfect revelation in Jesus Christ. And the Bible is the divinely-inspired testimony to the person and work of Christ.
(b) Salvation history. All that God has to do, and say, in connection with the bringing of salvation to the world, has been done and said.
(c) Scripture itself. Time and again the writers of Scripture draw attention to the finality of the message they have been given, cf Jer 23:16-18. Christ himself asserted strongly that the word of God could not be supplemented by the traditions of men, Mk 7:8. People who attempted to do so were nullifying the word of God. See also Gal 1:8; 2 Cor 11:4.
The history of the church does, of course, demonstrate the development of doctrines such as the Trinity and the incarnation. But these are derived from Scripture, and are of a completely different order from those which have been asserted independently from Scripture, such as the various Roman Catholic dogmas concerning the pope, the mass, and the Virgin Mary.
It is an unhappy characteristic of those who hold to the equal authority of their denominational traditions and Scripture that in practice they demand greater loyalty to those traditions than they do to Scripture. So some Anglicans are dogmatic about the threefold order of ministry and a particular doctrine of the eucharist, while maintaining scepticism about the incarnation or atonement.
The implication of this is that the preaching and teaching of the Bible will be the best form of evangelism, Jn 20:31; of building up the saints, 2 Tim 3:16-17; and of preparing people for the work of ministry.
Based on Peter Adam, ‘The Preacher and the Sufficient Word’, in When God’s Voice is Heard, 27-42.