Justin Taylor: https://www.thegospelcoalition.org/blogs/justin-taylor/one-rule-to-rule-them-all-the-real-meaning-of-sola-scriptura/
Robert Letham, Systematic Theology (Wheaton, IL: Crossway, 2019), 234:
[There is a] false notion, held widely, that the slogan sola Scriptura means that the Bible is the only source for theology. . . .
The slogan itself, still less the reality to which it pointed, never meant that the Bible was the only source for theology. The dangers of such a position are most clearly seen in the Socinians, the Jehovah’s Witnesses, and in the early Plymouth Brethren, who, in their first decade, recapitulated many of the heresies of the early church.
When the slogan [sola Scriptura, Scripture alone] was devised, it was never intended to exclude the tradition of the church.
Oliver Crisp, God Incarnate: Explorations in Christology (London: T&T Clark, 2009), 17, discusses the various levels of sources and authority in Christian theology:
[First-Order Authority: Scripture: The Rule that Rules]
1. Scripture is
- the norma normans [the norm of norms, the rule that rules—a norming, adjusting, or measuring standard by which other measuring tools are to be measured],
- the principium theologiae [fundamental principle/foundation of theology].
It is the final arbiter in matters theological. . . . the first-order authority in all matters of Christian doctrine.
[Second-Order Authority: Ecumenical Creeds: Rules that Are Ruled]
2. Catholic creeds, as defined by an ecumenical council of the Church, constitute a first tier of norma normata [a norm that is normed, a rule that is ruled, a standard or measure that is itself subject to, and defined by, a greater standard] which have second-order authority. . . .
Such norms derive their authority from Scripture to which they bear witness.
[Third-Order Authority: Confessional Statements: Rules that Are Ruled]
3. Confessional and conciliar statements of particular ecclesiastical bodies are a second tier of norma normata, which have third-order in matters touching Christian doctrine.
They also derive their authority from Scripture to the extent that they faithfully reflect the teaching of Scripture.
[Doctrines from Theologians: Legitimate Theological Opinions]
4. The particular doctrines espoused by theologians including those individuals accorded the title Doctor of the Church which are not reiterations of matters that are de fide, or entailed by something de fide, constitute theologoumena, or theological opinions, which are not binding upon the Church, but which may be offered up for legitimate discussion within the Church.