A 'rational' explanation is one that any other human of similar intelligence could observe and deduct (think) and come to the same conclusion. Rational comes from reason meaning: the power of comprehending, inferring, or thinking especially in orderly rational ways and/or a sufficient ground of explanation or of logical defense; especially : something that supports a conclusion or explains a fact. Either way no two independent observers are going to arrive at something like Genesis, and the fall, the redemption, the life of Christ, and all that based on looking at the world.
It's not that's it's good or bad, it's that the province of religion is faith, not reason. No doubt in my mind that people looked out on the world and lacked answers, so they made some up. Jesus, turtles, whatever. It may or may not be the truth. Science by the way does not say it has the truth, all science has is the numbers on what works what percentage of the time. That's all science is, a collection of things that work. We often realize it works long before we arrive at the 'reason'.