Traditionally, the doctrine of the Trinity has carried the mystery card. But compared to the doctrine of the Atonement, I think the Trinity is basic arithmetic (hmm, maybe that’s the wrong analogy here).
The doctrine of the Trinity–its development, formalization, and classical and contemporary expressions–can be understood with little intellectual toil. It’s basically a matter of coming to grips with important theological terms, concepts, and basic metaphysical distinctions. And, importantly, the Biblical data seem consistent with a wide variety of models of the Trinity.
The doctrine of the Atonement, on the other hand, is much less straightforward. The theological terms at its heart (e.g., “ἱλαστήριον”) are puzzling, no one model seems to give due justice to the central theological concepts (e.g., victory, satisfaction, substitution, etc.), and how the ethical machinery works is far from obvious (e.g., transfer of moral guilt). Every model seems to capture something deep and profound about the atonement, but it is exceedingly hard to see how to piece them all together to form a coherent, unified picture.
Further, it’s much harder (at least for me) to approach the doctrine of the Atonement in an intellectually dispassionate manner. Most of the time I can puzzle over the concepts, terms, and the logic of the Trinity as cooly as I can a math or science problem (most of the time). But when I try to parse out the Atonement, I get the overwhelming feeling that there’s something profoundly deep and mysterious here, and we’re grasping at straws. If there’s any area of Christian doctrine that deserves to carry the mystery card, it’s the Atonement.
There’s Deep Magic in Narnia. But there’s the even Deeper Magic from before the dawn of time.