How do we know God is who He says He Is?
We know He is who He claims to be because He’s done what He said He would do. That’s the whole point of prophecy and why studying it is so important to us. It’s God authenticating Himself.
For 6000 years He’s been describing things before they happened, so when they did His people would know that He was telling us the truth. Everything He said would happen in the past has happened, so it makes sense to believe that everything He said would happen in the future will happen.
No other so-called Holy Book offers this kind of authentication. In fact in Isaiah 44:6-8 God challenged anyone who claimed to be like Him to a two part test. Part 1 is to accurately recount the past and part two is to accurately proclaim the future. Only God, who knows the end from the beginning can do this.