Recently I was asked by a Roman Catholic friend how I could escape Liberal and Unitarian arguments with sola Scriptura? I thought that was an excellent question! Here is my answer:
Both Liberalism and Unitarianism deny sola Scriptura by denying the sufficiency of Scripture, which is essential to a proper understanding of sola Scriptura. In other words, we may place both aberrations on the poles of a continuum defined by Scriptural sufficiency.
On one end is the denial of Scriptural sufficiency due to the denial that Scripture is our only infallible rule in matters of faith and practice (i.e. Liberalism). In this theory other authorities are accepted as infallible. In other words, Scriptural authority is supplemented with something else.
On the other end is a denial of Scriptural sufficiency due to the denial that Scripture was/is sufficient as our only infallible rule in matters of faith and practice in the same way over time (i.e. Unitarianism). In other words, the sufficiency of Scripture is denied for past generations of interpreters, completely undermining the authority of tradition (unless of course they were Unitarian!). This is sometimes referred to as solo Scriptura as opposed to sola Scriptura.
Of course there will always be difficulties with respect to the unity of the church. The question is not whether sinners will have difficulty confessing and living according to the truth of God's self-revelation. That is a given. Roman Catholics and Reformed catholics just resolve that tension differently.
Roman Catholicism posits epistemological religious certainty in an essentially perspicuous and self-authenticating Apostolic office (i.e. the Roman see). Reformed catholicism posits epistemological religious certainty in an essentially perspicuous and self-authenticating Apostolic teaching (i.e. the completed canon of Scripture).
Either way, at the end of the day Roman Catholics and Reformed catholics are trusting in someone else. We are slaves in need of a good master.
Roman Catholics have found that master in the Apostolic office, which they trust continues to the end of the age. Reformed catholics, on the other hand, have found that master in the God-breathed Apostolic teaching, the foundation of the church completed during the time of the Apostles (Matt. 16:18; 18:18; Eph. 2:20; Jude 3).