This Tradition, as it has historically been called, in addition to ‘filling in the gaps’ where Scripture is silent, also helps to explain and interpret Scripture for us, to assist us in arriving at the correct understanding of what the Scripture means. For we see the pernicious effects of using Scripture as the sole rule of faith and doctrine all around us in the divisions which plague the Body of Christ referred to above. These problems are nothing new to Christendom and gave rise to the famous test of sound doctrine coined by Vincent of Lerins in the 5th century: "Quod ubique, quod semper, quod ab omnibus" - "that [which has in relation to Christian doctrine been believed] everywhere, always, by everyone." In fact, I think that Vincent came out with a number of excellent points which are worth quoting in expanded format here from his Commonitory (here, ‘catholic’ simply means ‘universal’):-
“I have often then inquired earnestly and attentively of very many men eminent for sanctity and learning, how and by what sure and so to speak universal rule I may be able to distinguish the truth of catholic faith from the falsehood of heretical pravity; and I have always, and in almost every instance, received an answer to this effect: That whether I or any one else should wish to detect the frauds and avoid the snares of heretics as they rise, and to continue sound and complete in the catholic faith, we must, the Lord helping, fortify our own belief in two ways; first, by the authority of the Divine Law, and then, by the Tradition of the catholic church.
"But here some one perhaps will ask, Since the canon of Scripture is complete, and sufficient of itself for everything, and more than sufficient, what need is there to join with it the authority of the Church's interpretation? For this reason,—because, owing to the depth of Holy Scripture, all do not accept it in one and the same sense, but one understands its words in one way, another in another; so that it seems to be capable of as many interpretations as there are interpreters. For Novatian expounds it one way, Sabellius another, Donatus another, Arius, Eunomius, Macedonius, another, Photinus, Apollinaris, Priscillian, another, Iovinian, Pelagius, Celestius, another, lastly, Nestorius another. Therefore, it is very necessary, on account of so great intricacies of such various error, that the rule for the right understanding of the prophets and apostles should be framed in accordance with the standard of ecclesiastical and catholic interpretation."
So after stating his belief in the (material) sufficiency of Scripture, Vincent argues that the Church's standard interpretive Tradition is necessary because of the various ways in which different people (particularly heretics) have misinterpreted the Scriptures. To make this especially relevant to the situation today, and why the consensus of Tradition is necessary, one can substitute modern-day denominations and see how his argument still holds:
"For this reason,—because, owing to the depth of Holy Scripture, all do not accept it in one and the same sense, but one understands its words in one way, another in another; so that it seems to be capable of as many interpretations as there are interpreters. For Luther expounds it one way, Calvin another, Zwingli another, Methodists, Baptists, Mennonites, another, Adventists, Pentecostals, Presbyterians, Unitarians, Quakers, Campbellites, Plymouth Brethren, another, lastly, Jehovah Witnesses another. Therefore, it is very necessary, on account of so great intricacies of such various error, that the rule for the right understanding of the prophets and apostles should be framed in accordance with the standard of Ecclesiastical and Catholic interpretation."
Vincent concludes by laying down the criteria for interpretation:
"Moreover, in the catholic church itself, all possible care must be taken, that we hold that faith which has been believed everywhere, always, by all. For that is truly and in the strictest sense "catholic," which, as the name itself and the reason of the thing declare, comprehends all universally. This rule we shall observe if we follow universality, antiquity, consent. We shall follow universality if we confess that one faith to be true, which the whole Church throughout the world confesses; antiquity, if we in no wise depart from those interpretations which it is manifest were notoriously held by our holy ancestors and fathers; consent, in like manner, if in antiquity itself we adhere to the consentient definitions and determinations of all, or at the least of almost all priests and doctors.
What then will a catholic Christian do, if a small portion of the Church have cut itself off from the communion of the universal faith? What, surely, but prefer the soundness of the whole body to the unsoundness of a pestilent and corrupt member? What, if some novel contagion seek to infect not merely an insignificant portion of the Church, but the whole? Then it will be his care to cleave to antiquity, which at this day cannot possibly be seduced by any fraud of novelty.
But what, if in antiquity itself there be found error on the part of two or three men, or at any rate of a city or even of a province? Then it will be his care by all means, to prefer the decrees, if such there be, of an ancient General Council to the rashness and ignorance of a few. But what, if some error should spring up on which no such decree is found to bear? Then he must collate and consult and interrogate the opinions of the ancients, of those, namely, who, though living in divers times and places, yet continuing in the communion and faith of the one Catholic Church, stand forth acknowledged and approved authorities: and whatsoever he shall ascertain to have been held, written, taught, not by one or two of these only, but by all, equally, with one consent, openly, frequently, persistently, that he must understand that he himself also is to believe without any doubt or hesitation."
That there are folks who have mutually contradictory interpretations--on what the Scriptures seem to teach on such vital issues as the nature of God and the nature of Christ--should not be surprising. The Apostle Peter warned in his Second Epistle that there were already those who were "twisting Scripture" to "their own destruction". The question is how do we know: (1) who are the ones "rightly dividing the word of truth", and (2) who are the ones "twisting Scripture to their own destruction"? For each group, as I have said on the first page, is convinced that they are practising the former, while those who disagree are potentially practising the latter. Who decides between them, and/or how does one know who is right?
I submit the answer to the "how" question lies in the Apostolic Tradition. This is mentioned by Paul, particularly to the church of the Thessalonians:
"Therefore, brethren, stand fast and hold the traditions you were taught whether by word or our epistle." (2 Thess 2:15)
Notice that Paul doesn't elevate one mode of receiving the Tradition--his oral word or his written epistle--above the other; the authority is the same. Though we can't be sure that the content is exactly the same in every single detail of the oral and written forms, we can surmise that they testify to the same material Truth and therefore don't contradict one another. From another one of Paul's letters, this time to Timothy, it seems that this oral Tradition at the very least refers to (and consists of) the specific body of teaching and doctrine that was handed down by the Apostles:
"Hold fast the sound pattern of words which you have heard from me." (2 Tim 1:13)
So, there was a "sound pattern" of oral teaching recognizable to Timothy (and presumably to the others taught by the Apostles) which was to be kept and by which the early Christians could recognize truth from error. By this "sound pattern" the early Christians could therefore "rightly divide" the word of truth. On the other hand those who did not hold fast the "sound pattern of words" received orally from the Apostles could be considered "untaught and unstable" (2 Peter 3:16) and were liable to misinterpret the Apostle's writings (and the other Scriptures) and thus to "twist the Scriptures to their own destruction".