"Surely this important principle is itself written down somewhere?"
John 20:31, "But these are written, that ye might believe that Jesus is the Christ, the Son of God; and that believing ye might have life through his name."
1 Peter 1: 9-12, "Receiving the end of your faith, even the salvation of your souls. Showing that salvation in Christ is a thing prophesied of old of which salvation the prophets have enquired and searched diligently, who prophesied of the grace that should come unto you:searching what, or what manner of time the Spirit of Christ which was in them did signify, when it testified beforehand the sufferings of Christ, and the glory that should follow.Unto whom it was revealed, that not unto themselves, but unto us they did minister the things, which are now reported unto you by them that have preached the gospel unto you with the Holy Ghost sent down from heaven; which things the angels desire to look into."
2 Peter 1: 16-21, "For we have not followed cunningly devised fables, when we made known unto you the power and coming of our Lord Jesus Christ, but were eyewitnesses of his majesty. For he received from God the Father honour and glory, when there came such a voice to him from the excellent glory, This is my beloved Son, in whom I am well pleased. And this voice which came from heaven we heard, when we were with him in the holy mount. We have also a more sure word of prophecy; whereunto ye do well that ye take heed, as unto a light that shineth in a dark place, until the day dawn, and the day star arise in your
hearts: Knowing this first, that no prophecy of the scripture is of any private interpretation. For the prophecy came not in old time by the will of man: but holy men of God spake as they were moved by the Holy Ghost."
2 Tim. 2: 14-16, "Of these things put them in remembrance, charging them before the Lord that they strive not about words to no profit, but to the subverting of the
hearers. Study to shew thyself approved unto God, a workman that needeth not to be ashamed, rightly dividing the word of truth. But shun profane and vain babblings: for they will increase unto more ungodliness."
2 Tim. 3: 14-17, "But continue thou in the things which thou hast learned and hast been assured of, knowing of whom thou hast learned them; And that from a child thou hast known the holy scriptures, which are able to make thee wise unto salvation through faith which is in Christ Jesus. All scripture is given by inspiration of God, and is profitable for doctrine, for reproof, for correction, for instruction in righteousness: That the man of God may be perfect, throughly furnished unto all good works."
Acts 17: 11-12, "These were more noble than those in Thessalonica, in that they received the word with all readiness of mind, and searched the scriptures daily, whether those things were so. Therefore many of them believed; also of honourable women which were Greeks, and of men, not a few."
Psalms 119.
That should be enough to demonstrate that "the scripture alone contains all that God has revealed for the salvation of man and fellowship with Him". Nowhere does the scripture say that there is a revelation outside of scripture that reveals God's plan of salvation for man or how sinful man can have restored fellowship with Him.
Nowhere does the scripture say that there is a revelation outside of scripture
Sure it does:
the Paraclete, the Holy Ghost, whom the Father will send in my name, he will teach you all things, and bring all things to your mind, whatsoever I shall have said to you. [...] 30 I will not now speak many things with you.(John 14)
Note the future tense.