i also know that unless that the word and the testimony was given, you could not have heard it, and could not have received it if you had not heard it.
note the bit that says "He shall not speak of Himself"
so we know if a spirit glorifies itself, it is not from God, but the Spirit of Truth glorifies God, and testifies of His Son.
now we have two witnesses: the Spirit sent to us by our Savior, and the scripture that testifies of Him. if we ignore either, the truth is not established.
i don't think it's wise to be flippant about the scripture - see how it causes my brother to stumble?