Of the beliefs and practices whether generally accepted or publicly enjoined which are preserved in the Church, some we possess derived from written teaching; others we have received delivered to us "in a mystery" by the tradition of the apostles; and both of these in relation to true religion have these same force…We are not, as is well known, content with what the Apostles or the Gospel has recorded, but both in preface and conclusion we add other words as being of great importance to the validity of the ministry, and these we derive from unwritten tradition…Does not this come from that unpublished and secret teaching which our fathers guarded in a silence out of the reach of curious meddling investigation?…What the uninitiated are not even allowed: to look at was hardly likely to be publicly paraded about in written documents?…In the same manner the Apostles and Fathers who laid down laws for the Church from the beginning thus guarded the awful dignity of the mysteries in secrecy and silence…This is the reason for our tradition of unwritten precepts and practices, that the knowledge of our dogmas may not become neglected and contemned by the multitude through familiarity (§66). On the Holy Spirit: St. Basil the Great (Popular Patristics Series Volume 42), Stephen M. Hildebrand, trans. (St Vladimir's Seminary Press, 2013), 133-136.
This presents an alternative to sola Scriptura. What's striking about it is Basil's explicit appeal to secret tradition. His justification of secret tradition. Traditions that are transmitted through covert channels. Only church authorities are privy to that "mystery".
But an obvious problem with sub rosa tradition, with a process that's deliberately shielded to public scrutiny, is the fact that there's no check on fabricated dominical or apostolic traditions, or legendary embellishment. Imagine the unbridled power that confers on church authorities. They can stipulate dominical or apostolic pedigree for a particular tradition, and if you refuse to credit that tradition, then you are rebelling against God himself.
Compare that to Scripture, which is a public record. In the early church, moreover, there was no controlled transmission of Scripture. Christian scribes informally produced copies of the Bible. It wasn't possible for church authorities to modify Scripture, to slip a fabricated dominical or apostolic tradition into the text of Scripture, for the chain of custody was in the public domain. The early church didn't have the centralized command and control to alter the record of Scripture. Individuals might attempt that, but would be unable to universalize their additions. Scripture presents a common frame of reference, precisely because that was in writing, and the transmission wasn't coordinated.
To take a comparison, suppose you were a 1C Christian member of a church in Asia Minor that St. John planted. Suppose you sat under his teaching. You'd have a duty to remain faithful to what he taught you, to the best of your recollection.
Now, to change the hypothetical, suppose somebody tells you, "This is God's will!" And you ask, "How do I know that's God's will?" And he says, "I sat at the feet of St. John, and here's what he said!"
That's a very different situation. If you submit to his claim, that person has absolute power over you. He's virtually God's mouthpiece, via his claim to be reproducing St. John's teaching.
But, of course, it would be very convenient for a heretic to adopt that imposture. By contrast, 1 John supplies a public frame of reference. That's available to Christians generally. Indeed, St. John wrote it for popular consumption. Every Christian with access to 1 John has the same standard of comparison.