It might be proof for the person who God spoke to, but certainly not proof for anyone else. We have these people all the time on this forum. People who claim to hear from God are convinced they have the truth. But, we don’t accept that as evidence. I do find it odd that posters are now suggesting God directly speaking to people is the optimal way for God to convey his message when we don’t accept that on this forum. I’ve yet to see anyone on this forum accept the testimony of a person saying he or she heard from God as acceptable evidence.
What I believe is more convincing is a written document that multiple people can attest to. An example of this is Wikipedia. It’s not controlled by any official group and open to input from the general public. But we consider it pretty good evidence to use in arguments. Another example are peer reviewed articles. Because they have multiple people attesting to it, it carries more weight than an unreviewed article. In my line of work, any code written has to be reviewed by peers in order to make it into production. Because multiple people attest to the code, it’s more likely to be better code.
On the other hand if the purpose of God’s communication were to generate confusion and misunderstanding, to force humans to work it out for ourselves whether as some kind of ant-farm experiment or as some kind of learning process important to our development as individuals or as a species, then a tome full of diverse theologies and tales of dubious provenance might be exactly the sort of thing to include in the mix!
I believe one of the main purposes of the Bible is to learn how to have a relationship with God. I would hope Christians of all stripes would agree on this. I would also hope Christians agree the summary of the Bible is to love God and love others. Beyond that, yes, we have differences in theology.
In a way, the Bible is simple to understand. A child can understand the Bible. Yet, the message is also quite complex so that a PhD can study for a lifetime and still not understand. I do not believe God’s intention is to make things obvious, rather I believe God’s intention is to reward those who seek him. “And without faith it is impossible to please him, for whoever would draw near to God must believe that he exists and that he rewards those who seek him.” (Heb 11:6)