“I take that as because scripture says so.
You should take it as exactly what it says, scripture being “profitable for doctrine, reproof, correction, instruction in righteousness, so that the man of God may be perfect, throughly furnished unto all good works” because it is “inspired” by the Holy Ghost. Seems a pretty good basis for doctrine then for the Christian.
How did you come to decide that Timothy in the example and other writings were truly scripture and truly inspired by the Holy Ghost?