Board Thread:Rebirth: Applications/@comment-8914761-20141019201347/@comment-8914761-20141101212208

Well, from what I know, and from what I've drawn up, the Rinnegan appears in someone when the world is in some sort chaos. But let's just throw that aside. The rather than preaching peace as an overall rule, preaches about ones/overall Inner Peace. Which was exactly what his mother did through her religion. Sigma's inner peace presented itself in the form of the Rinnegan, he connected with something deep inside his body, soul, and spirit.

To answer your question in short: Sigma's Rinnegan manifested because instead of purely practicing the religion, he instead applied it to his own self. As the terror of BC would begin, the Rinnegan would appear in Sigma, specifically because of his way of the Dharma and finind his peace.