should we believe your teachings when you say that you are being led by the Spirit
or should we believe Bob's contrary teachings when Bob says that HE is being led by the Spirit
or should we believe Jill's teachings - which are contrary to both yours and Bob's - when Jill says that SHE is being led by the Spirit...
or should we "test the spirits"?
And...how should we test them
other than by evaluating their teachings by comparison to Scripture, which "is given by inspiration of God, and is profitable for doctrine, for reproof, for correction, for instruction in righteousness..."
You seem to think that Paul was speaking of writing in general when he used the term "letter" in that passage...
but he was not. He was comparing the letter of the law to the spirit of the law
as Jesus often did
and pointing out that the old covenant (the letter of the law)
was not the same as the covenant that we follow, one of the spirit.
He was not condemning letters (written teachings)
but if he were
we would be - hopelessly - stuck with the quandary that I mentioned at the beginning of my answer.