"Throughout the whole period Scripture and tradition ranked as complimentary authorities, media different in form but coincident in content" (Early Christian Doctrines pp 47-48--emphasis mine--with the preceding pages giving specific citations in the writings of the Fathers as evidence of this perceived relationship).
In other words, the content of the body of doctrine/morals is basically the same (or coincident) though expressed in different forms--written epistles or narratives on the one hand, baptismal confessions, 'rules of faith', hymns, catechesis, and (later) creeds, etc on the other. All the above was considered to have it's source in the Apostolic teaching or Tradition of orally proclaiming the crucified and resurrected Jesus Christ as the fulfillment of the OT Scriptures and thus the Principle for correctly interpreting the same. As this Tradition--whether ultimately written down in the Scriptures or expressed in the hymns, confessions, 'rules of faith'--came from a common source (the Apostles) and expressed a common Truth (the Gospel kerygma), the early Christians considered it's various forms equally authoritative and thus complimentary. Thus Kelly continues...
"To inquire which counted as superior or more ultimate is to pose the question in misleading and anachronistic terms. If Scripture was abundantly sufficient in principle, tradition was recognized as the surest clue to its interpretation, for in tradition the Church retained, as a legacy from the apostles which were embedded in all the organs of her institutional life, an unerring grasp of the real purport and meaning of the revelation to which Scripture and tradition alike bore witness." (ibid. p 48).
So while the early Christians took it for granted that all of the Church's essential doctrines were contained in the Canon (once completed), they also recognized that these other media (ie "organs of her institutional life") conveyed the same Truth, albeit in different forms, and that these were thus helpful at arriving that the correct interpretation of the Scriptures against the distortions of the heretics. [You see], the heretics often would appeal to the same Scriptures to arrive at their false teachings, which was evident as far back as the Apostolic age (see 2 Peter 3:16), and it was often by an appeal to its devotional life (prayers/hymns), 'rules of faith', and common teaching that the Church authoritatively declared the true meaning of Scripture in opposition to heresy. This idea can be summed up in the statement that the Scriptures were believed to be materially sufficient (ie contained all the necessary "stuff") but formally insufficient (ie prone to misinterpretation if read outside of the context of the Church and the Apostolic Tradition.)
