Sentence:  The Stoic idea was based upon an astronomical doctrine according to which the return (<WSD>"apokatastasis</WSD>") of the planets to their proper "celestial signs" initiates the conflagration ("ekpurôsis"), which is the reduction of the entire cosmos to its primal element (fire), after which follows the rebirth of all existing things. 
Meaning of the word marked with <WSD> tags:  An apocalypse leading to the remaking of the world rather than a Final Judgment,  an Origenist heresy condemned by the 543 (A.D.) (Synod of Constantinople).