Let's look at what it means for the Bible to be authoritative. Should we accept every word as coming directly from God? Of course not. The OT histories are based on traditions from the vague past. Archaeology isn’t very kind to it, though there is reason to think that there’s some actual memory of those past times embedded in it.
But the Gospels are a lot closer to Jesus’ time, and seem to be based on documents that are even closer. Sure, the four writers interpreted it from different points of view, but we can certainly see Jesus there.
If we’re going to claim to be Jesus’ followers, we need some basis for knowing what that means. In the first century, you could talk to witnesses, or at least their disciples. But by our time, the Gospels are the only primary source for what Jesus actually said and did. That’s the basis for its authority. And I think it gives us enough to go on to form a pretty robust religion.
I’m not even so hostile to traditional Christianity. You can see quite clearly how Christianity was formulated to meet the needs of the time, and how the intellectual formulation was designed to let it be taken seriously by intellectuals at the time. But in many ways our metaphysics today is closer to the 1st Cent Jewish concepts than to 3rd and 4th Century Greek versions, and it’s not so clear that the reinterpretation to meet the needs of ancient culture is as useful to the modern portions of ours. (There are plenty of people for whom it still addresses issues that matter to them. But they’re not the ones that are leaving the faith.)
I think Jesus of the Synoptics, if combined with traditional spiritual practices and good worship, is perfectly viable in our culture. There are places that are making it work. But too often good scholarship is combined with insipid worship and a lack of emphasis on spiritual disciplines.