The apostle Paul says: “Therefore, brethren, stand fast, and hold the traditions which ye have been taught, whether by word, or our epistle” (2 Thessalonians 2:15, KJV)
This is some biblical evidence for the importance of holding non-biblical (i.e. non-epistle) beliefs.
The Protestant teaching of sola scriptura is not a heresy or a novel doctrine, but in reality it is a reaffirmation of the faith of the early church. It is both biblical and historical, yet the Roman Catholic Church continues to teach that oral tradition is a second source of divine revelation, equally as authoritative as Scripture and that this was the view held by the church Fathers. Such a claim, however, contradicts both Scripture and history. When the Fathers speak of a tradition handed down from the apostles independent of Scripture, they are referring to ecclesiastical customs and practices, never to doctrine. Tradition was always subordinate to Scripture as an authority, and the Word of God itself never teaches that tradition is inspired. The Scriptures give numerous warnings against tradition, ('See to it that no one takes you captive through hollow and deceptive philosophy, which depends on human tradition and the basic principles of this world rather than on Christ' (Col. 2:8); 'Thus you nullify the word of God for the sake of your tradition....They worship me in vain; their teachings are but rules taught by men.' (Matt. 15:6, 9; cf. Mark 7:3-13; Gal. 1:14; Col. 2:22; 1 Peter 1:18) and the Fathers rejected the teaching of an apostolic oral tradition independent of Scripture as a gnostic heresy. For the church Fathers apostolic tradition or teaching was embodied and preserved in Scripture. The teaching of the Fathers is this: What the apostles initially proclaimed and taught orally, they later committed to writing in the New Testament. Irenaeus succinctly states it in these words:
We have learned from none others the plan of our salvation, than from those through whom the gospel has come down to us, which they did at one time proclaim in public, and, at a later period, by the will of God, handed down to us in the Scriptures, to be the ground and pillar of our faith. (Irenaeus, Against Heresies III.1.1, in Alexander Roberts and W. H. Rambaugh, trans., in The Writings of Irenaeus (Edinburgh: T & T Clark, 1874)
How is one to know what the apostles taught orally? It has been handed down to us in the Scriptures, and they in turn are the ground and pillar of our faith. The historical circumstances that prompted Irenaeus's words are important to understand. He was writing against the Gnostics who claimed to have access to an oral tradition handed down from the apostles, which was independent of the written Word of God. Irenaeus, as well as Tertullian, explicitly repudiates such a concept. The bishops of the church were in the direct line of succession from the apostles, and they were faithful to the apostolic teaching they proclaimed orally, but that doctrine could at every point be validated by Scripture.