True sovereignty cannot be divided between multiple, individually distinct sources. No one can serve two masters, for either he will hate the one and love the other, or he will be devoted to the one and despise the other. (Matthew 6:24a) The dissemination of authority may branch out in various forms, but such stewards will be held responsible to the Origin of their power. That we may successfully uphold our responsibilities thereof, it is necessary to identify the means by which they are communicated to us, which must be similarly unified lest it be rendered ineffectual. This must be the Sacred Scriptures, the Holy Bible, not only because it is the very Word of God Himself (which would be enough), but also the only such authoritative communication He has seen fit to bestow upon us. I make these assertions confidently, though they are not generally received without controversy.
That the Bible is the Word of God is necessarily a truth transcendent, beyond our worldly means of verification. To objectively demonstrate its authority would require us to make it subservient to a superior source; but if it be the Word of God, then only God Himself is fit to verify it. Attempts to accumulate support for its authenticity fall short of true verification, for such inductive evidences are always subject to their experiential nature. Often the Holy Spirit testifies of His Word directly to the heart of the believer, convicting him of its truth; this is sufficient for the individual, but evades the selfishly minded inquirer. Ultimately, I assert its truth based upon this conviction, and trust that God convicts others according to His will.
Unfortunately, there are those who falsely claim further revelation from God beyond that of the Bible. A multitude of additional works exist purporting authority equivalent to that of Scripture, from supposedly lost ancient books of wisdom to additional accounts of Christ. The viability of such works is a topic of its own, but for now there are two points to consider: first, the authorship of these works are often in question, some even demonstrably of false claim; second, most such records espouse information directly contradictory to Scripture. In addition to widely circulated works, individuals will sometimes claim new prophesy for their immediate acquaintances. While this is more difficult to address for its personal aspect, the same principles technically apply; such revelation is not experientially falsifiable, and would be subject to the judgement of Scripture regardless.
All this brings us to the question of sufficiency. Even amongst those who supposedly accept the superiority and exclusivity of the Bible, there is still distention upon this principle. Theirs is usually an argument of practicality relating to interpretation; given the corrupted nature of man, consistent and accurate understanding of God’s Word cannot be accomplished without some additional standard as guidance. There is an element of truth to this, but it is no justification for elevating the wisdom of man to the level of being a judgement of Scripture. No matter how well-meaning or educated a person may be, they are as prone to human error as everyone else. The Bible itself recommends the accountability of the Holy Spirit and the edification of the Church as sufficient safeguards against heresy, as accounted in 1 Corinthians; therefore let us respect the Sovereignty of the one, and uphold the responsibility of the other.
Be sure of truth.