Probably Zoroastrianism. A strong Persia focused thus would tend to imply either a weakened Islamic spread or one that was focused in a different direction (more South into Africa or North into Europe).
Assuming Islam attempted to spread the same as IOTL, but was stopped at Persia, you then have its spread the East limited so Islam might be limited in India/Pakistan and on into Indonesia, etc. Depending on its post-failed-conquest relationship with Persia, Islam might have to contend with an unfriendly/neutral power nearby so might have less power to focus going West and North, so might have made less headway into Europe and might have had a bit less success in the Crusades.
So Persia not falling could potentially help keep the Byzantine Empire (at least a rump thereof) alive, might prevent some of the religious disputes that racked India later, etc.
One question might be whether, then, Zoroastrianism spreads into other areas (greater penetration in India, more presence in the Far East, etc).