But what about the successors of the apostles and the prophets? Aquinas taught that we believe them insofar as they teach the same things that the apostles and prophets taught:
All the intermediaries through which faith comes to us are above suspicion. We believe the prophets and apostles because the Lord has been their witness by performing miracles, as Mark (16:20) says: "...and confirming the word with signs that followed." And we believe the successors of the apostles and prophets only in so far as they tell us those things which the apostles and prophets have left in their writings.Latin text:
Ad undecimum dicendum, quod omnia media per quae ad nos fides venit, suspicione carent. Prophetis enim et apostolis credimus ex hoc quod Deus eis testimonium perhibuit miracula faciendo, ut dicitur Marc., cap. XVI, 20: sermonem confirmante sequentibus signis. Successoribus autem apostolorum et prophetarum non credimus nisi in quantum nobis ea annuntiant quae illi in scriptis reliquerunt.Citation: St. Thomas Aquinas, Truth, Vol. 2, Questions X-XX, trans., James V. McGlynn, S.J., Question 14, Article 10, Reply, Answer 11 (Chicago: Henry Regnery Company, 1953), p. 258.
Notice then that for Aquinas the Scriptures serve as the sort of outer markers for what those holding apostolic succession can teach and expect us to believe. Notice especially that it is not simply "which the apostles and prophets have left" but he adds "in their writings."