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".
What's more is that Paul expects Timothy to be able to transmit orally that which he received from Paul: "And the things you have heard from me among many witnesses, commit these to faithful men who will be able to teach others also." (2 Tim 2:2). Notice here that the "things" Timothy received were not exclusive or private but were heard "among many witnesses", and how the faithful men to whom Timothy committed these "things" were to teach others as well (that's four generations of oral transmission). The public reception of the Tradition in the community could thus serve as a "check" or "balance" on those who would deviate from the "sound pattern" while claiming (ie like the Gnostics did) to be handing down some new teaching allegedly received "secretly" from the Apostles.
As for the answer of "who decides?", it is obviously the CHURCH corporately. As Paul wrote to Timothy, the CHURCH is "the pillar and ground of truth". (1 Timothy 3:15). Christ commissioned His Apostles, the collective foundation of the Church (Eph 2:20), by breathing His Spirit, the Spirit of Truth (John 16:13), on them (John 20:22). The Apostles by the Spirit preached the Truth and established local congregations of the Church to whom they delivered the faith once for all (Jude 3) in "sound patterns of words" (2 Tim 1:13). Some of the Apostles by the Spirit also wrote inspired authoritative epistles and narratives to some of these same congregations. The Church in time, led by the Spirit, could finally come to a consensus on the limits of the Scriptural Canon--by determining which works conformed to the Tradition received ("sound pattern"), and those which, though claiming apostolic authorship, did not (ie Gnostic and Ebionite texts)
The Church (collectively), recipient of both the "sound pattern of words" and the Apostolic writings, could thus collectively judge truth from error. In fact, we see the Church doing just that even in those early years shortly after the Apostles left the scene. By the authentic Apostolic writings and the "sound pattern of words" (often later referred to as "the rule of faith"), expressed in hymns, catechesis, and short-summaries, the Church was able to determine what was heretical. So even in the ante-Nicene era (before Constantine allegedly "corrupted" and "counterfeited" the Church in the 4th century), the Church was able, for instance, to fend off false teachings such as claiming Jesus did not really come in the flesh (docetism), diverse teachings that matter was evil and that there were two (or even multiple) ‘gods’ (Gnosticism), that Jesus only became God’s Son at His baptism (adoptionism, principally propounded by Apollinarius ) and that the Trinity was really just one god expressed in three ways or ‘modes’ (modalism, put forward by Sabellius ) and authoritatively declare such teachings "heresy" based on the Tradition received from the Apostles. And in the Nicene era, when the orthodox party and the Arians were constantly throwing Scriptural proof texts back and forth at each other, it was on the basis of received Tradition that the Church was able to convict the Arians of "twisting the Scriptures" by teaching falsely concerning Christ. In short, the Church was able to determine which were heretical practices and beliefs because it was able to say, in effect, “Hang on a minute, this isn’t what we’re used to; we’ve never thought or done that before.”