According to you're statement you would believe any writing that claims to be the word of God.
You need an authorized authority to put any such claim to the test.
That Authority is the Catholic Church historically established by Jesus Christ which has proved it's Divine origins by the fact that it has existed for 2000 years unchanged, as Christ predicted.
I agree with that to a degree. If I wish to establish the authenticity of a document, I need proof, don't I? If I have a writing that claims to be "Letter to the Ephesians from Paul the Apostle" I need some evidence to verify that.
Happily for us, that evidence was presented by those who had had chain of custody on that letter from Paul, and therefore, it was accepted into the canon of scripture.
Now for part 2.
Once I've agreed that these are the words of the Apostle Paul, my part is to do what: (1) pretend I'm in charge of those words, or (2) Acknowledge the authority of those instructions of an Apostle over me?
No matter how we approach it, we end up with the unique authority of scripture.
That is not to say that there isn't authority in the church for there is. However, we must admit that the Bible ONLY contains the inspired Word of God through His prophets and His apostles.