If ideas of Christ/God/etc. were truly independently reasoned into by someone never exposed to Christianity (as anything else would not satisfy any definition of "independent"), it would really be a miracle. However, it is much more likely to be a post-rationalisation.

Yes, conceptually similar philosophical ideas could be reached from first principles, but those would not come in the same exact packaging as Christianity (and it is packaging that makes many religions not that different from cults preying on vulnerable people).