Full Idea
I don't know how to demonstrate that God cannot annihilate the soul, but only that it is entirely distinct from the body, and consequently that it is not naturally subject to die with it, which is all that is required to establish religion.
Gist of Idea
I can't prove the soul is indestructible, only that it is separate from the mortal body
Source
René Descartes (Letters to Mersenne [1640], 1640.02.24)
Book Reference
Descartes,René: 'Philosophical Essays and Correspondence', ed/tr. Ariew,Roger [Hackett 2000], p.92