During a significant part of history, text was definitely considered a source of truth, at least to the extent a lot of people see video now. A fancy recommendation letter from a noble would get you far. It makes sense because if you forge it, that means you had to invest significant amount of effort and therefore you had to plan the deception. It's a different kind of behavior than just lying on a whim.

But even then, as nowadays, people didn't trust the medium absolutely. The possibility of forgery was real, as it has been with the video, even before generative AI.