Better yet, how do you know any modern religion is anything like what it should be like, generations later?
Religions seem very sure about their own teachings, even as they change. Within your own lifetime you’ve probably noticed that a priest or simply a believer you know has ended up changing their mind on something. Just a generation or two of believers and the current ones won’t be thinking and saying the kind of stuff the first ones were, and vice versa.
One pope says nay, next one says yay. If god is speaking through them, did god change his mind? If he is, why didn’t he just get it right from the start?
Religion isn’t like logic, which states 2+2 will always be 4. The simple passage of time and the broken telephone that is human word of mouth, means religion is incapable of staying consistent for more than about a decade, if that.
What’s more, the religions that exist today are the ones that were the best at spreading. If a religion isn’t appealing, people don’t stick with it. So religions tend to morph and splinter, evolving into whatever is just nice enough that a bunch of people will sign up.
They are the original meme, in the scientific sense. An infectious idea that gets recounted over an over, each person changing it slightly to be more appealing during a re-telling, empowering its spread.