abiding therein khālidīna fīhā is a circumstantial qualification of a future status in other words their abiding therein will have been ordained once they enter it — a promise of God in truth that is to say God promised them this and realised it in truth; and He is the Mighty Whom nothing can overwhelm and so prevent Him from fulfilling His promise and His threat of chastisement the Wise the One Who assigns all things to their rightful places.