In many formulations of Sola Scriptura, we are told that the Bible is formally sufficient when it comes to matters of salvation. Further, we are told that doctrines that do not affect salvation can never be necessary to be believed under pain of sin and even heresy. On this, note the following from Robert Bellarmine in his interaction with John Calvin:
Calvin says that the
famous proposition, “The Church cannot err,” is true with a two-fold
restriction. 1) If the Church does not propose doctrines outside of Scripture, i.e.
if it rejects traditions not written and only faithfully proposes what is
contained in the Scriptures. Moreover, if you ask whether we might be certain
that the Church always faithfully proposes those things that are in the
Scriptures, Calvin responds by applying a second restriction, the Scriptures in
matters necessary to salvation, still not in other matters and consequently
some blemishes of error always remain in the Church.” (Robert Bellarmine, De
Controversiis: Tomus II On the Church, Volume 1: On Councils, on the Church
Militant, On the Marks of the Church [trans. Ryan Grant; Post Falls, Idaho:
Mediatrix Press, 2017], Book 3 Chapter XIV, p. 321)
In response to Calvin’s belief in the
formal sufficiency of Scripture and the Church not being authorised to propose doctrines
outside of Scripture, Bellarmine wrote the following, which I found to be an
excellent counter to Sola Scriptura advocates even today:
If Calvin’s opinion
were true, then a great part of dogmas of faith could be called into doubt, for
there are many de fide teachings which are not absolutely necessary to
salvation. Duly, to believe in the histories of the Old Testament, or that the
Gospels of Mark and Luke are canonical writings, nay more than any of the
Scriptures, is not altogether necessary for salvation, since without this faith
many were saved before the Scriptures were read; afterward, in the time of the
New Testament, many barbarian nations were saved without them, as Irenaeus
writes ([Against Heresies] lib. 3, cap. 4). But this is most absurd, nor would
Calvin admit there can be any doubt about Scripture, therefore it is not true
that the Church cannot err only in those matters necessary for salvation.
(Ibid., 324)