Full Idea
It is quite obvious that a perfect God cannot be a deceiver, for it is manifest by the light of nature that all fraud and deception depend on some defect.
Gist of Idea
It is self-evident that deception is a natural defect, so God could not be a deceiver
Source
René Descartes (Meditations [1641], §3.52)
Book Reference
Descartes,René: 'Discourse on Method/The Meditations', ed/tr. Sutcliffe,F.E. [Penguin 1968], p.131