Paul received direct revelation from the Lord, and most of what he wrote was written directly as Scripture. And while God also inspired the actual writing of Truths from oral tradition, this not only does not mean that those orally passed-down truths were all given under plenary inspiration of God (any more than in the case of quoting pagan prophets), but it also cannot mean that all that is passed down orally is wholly inspired of God.
The reality is that oral tradition is by nature unreliable, being supremely subject to corruption, while as can be more shown, God manifestly made writing His most-reliable means of authoritative preservation.
And yet since some of Scripture consistent of teachings and records that were passed down, orally or in some form of writing, the issue is on what basis are those who be accepted as being from God, versus all that claims to be from God, whether orally passed down or in writing.
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