Experience. I've had many theological discussion with Christians, not only on this forum, but IRL, and was raised among them. I have encountered maybe a handful on the internet if that, that do not believe this literally. I have yet to encounter a Christian IRL that doesn't believe this literally.
I've met quite a number of them, and attended a Christian church where none of them were literalists and even included atheists among them. I've also attended a Christian church where all of them were literalists. But beyond personal anecdotal evidences, the fact the authors like Borg, Crossan, Mack, Pagels, Spong, et. al, continue to publish Christian books for modern Christians as well as other interested parties alike for the past several decades, would seem to indicate there's more than just a handful of Christians who are no longer in that premodern, mythic-literal reality.
In fact, James Fowler's research found in his book Stages of Faith (which I've read through multiple times), shows that there are 4 stages beyond mythic-literal beliefs (Stage 2), though you could hold the Stage 3 is just tacitly accepting literalism without really challenging it, but not rigidly so. So at least 3 full stages beyond literalism and a premodern reality.
The statistical distribution of these higher stages beyond the literal stage, are substantial enough in the population that he researched, that he was able to map out a pattern clearly distinguishable from the earlier stages. That means, it's objectively real, it's not rare, nor unheard of at all.
This is a summary of Fowler's stages, and in looking for them, you'll see almost all of them represented right here on RF, just as they are IRL, as Fowler's research shows.
Handout 1: Stages of Faith Development
I would place myself largely into the Stage 5 faith category where I am at now today, yet still struggling with an allergy to organized religion in general. It's all part of it. But I am at a place I am able to reintegrate some of the earlier important symbolisms, into a broader, more inclusive and comfortable acknowledgement of inner truths, beyond the literalistic perspectives, which clearly have an issue when running into a more rationality-based perspective. Reason is no longer an enemy of faith, but a set of eyes to help keep it grounded in reality.
