Yes, the belief that we are created in the image of God.
Following along Christian beliefs, since speech is instinctive, therefore "in the image of God," and writing is an invention which occurred after The Fall, I can deduce that writing is not in the image of God, and as an invention of Fallen Man, not to be trusted.
I'm making a distinction between how we were created, according to Christian belief, and the fact that we aren't created with an instinct to read and write.
Yes, I get that. It's irrelevant to the argument. I have no way of testing this other than the very written word you use as its own test. I can test whether thought and speech are created in the image of God. We each are endowed with thought and speech. No one has to be taught to think and speak. I don't need any written words to know this.
Reading and writing are not innate to the created human being. Whether you believe writing is a secret revealed by a lesser god is irrelevant. People still need to be taught to read and write. The chain of education is linked with and by human beings. If we separate a group from this chain at a pre-linguistic age, isolate them, and raise them, they each will grow up thinking and speaking. After several generations, language would evolve. None would ever write. Unless and until the idea of recording thoughts with symbols occurs to one of them, they will go on, created in the image of God, without the invention of writing.
Thinking and speaking are innate to God's human creation. Writing is not.