It is my understanding that we have a single deposit of faith (divine revelation) delivered from the Word of God himself (Jesus) to his apostles during life and secondarily from the apostles - after his death and resurrection - under divine inspiration, which is preserved in the life of the Church chiefly through the apostolic succession of the episcopacy under two modes: written Sacred Scripture and unwritten Sacred Tradition, with the Magisterium being the authoritative interpreter of the deposit.
I’m having difficulty, however, in fathoming how some Catholics - including Catholic theologians of note (such as Yves Congar and it is claimed a number of the Fathers) - arrive at a belief in the “material sufficiency” of scripture, even as they admit that it still forms a subsection, albeit the most singular and primary subsection, of the Church’s Tradition.
As I understand their thinking in this regard, it is held that Scripture and Tradition both contain the fullness of divine revelation “in total”, with the Tradition acting as the inspired interpreter of the former (i.e. ensuring that it is orthodox), such that one can say everything ‘sufficient’ to salvation can be found in Scripture alone. This is in contrast to the Partim-Partim approach that was most favoured around the time of the Council of Trent and defended by St. Robert Cardinal Bellarmine, which holds Scripture and Tradition to be co-equal modes of authority deriving from the one deposit of faith, both of which contain ‘constitutive’ doctrines that are essential for salvation - such that Scripture would not be “materially sufficient” in this interpretation, because Tradition preserves oral teachings not found in Scripture.
Doubtless, the ecumenical advantage of the ‘material sufficiency’ theory is not lost on me (in finding common ground with our Protestant brethren) - but I honestly struggle to view it as a tenable proposition in and of itself.
On the contrary, I personally think there are truths of the faith - including substantial truths of the faith - which are neither explicit nor even reliably implicit in Scripture but which the Church has held from Apostolic times to be authoritative teaching.