neosaiyan is almost correct.
clouds infusion with the jenova cells is what gave him geostigma.
sephiroths infusion made him a god because of when it happened, his body COMPLETELY fused with the cells. with NO rejection.
hash, i see where your coming from, but you have missed a few key parts. for instance, in the game, you only fight lesser versions of sephiroth. the REAL sephiroth, has only just awakened during the final battle, and isnt up to his full potential, and yet, he can still summon suprnova heh. hes already mastered the ability to shapeshift (inherited from jenova) and lets face it, even though cloud destroyed his body, he already made it clear he could continue to exist afterward.
the only thing that stopped sephiroths plan, was Holy, cloud had nothing to do with that, Airis summoned Holy to stop Meteor. if anything, cloud had done sephiroth a favour, by destroying his body, returning him to the lifestream meant that sephiroth could easily absorb its energy, and devour the energy of those destroyed by meteor when the time came.