Full Idea
That the things we grasp very clearly and very distinctly are all true, is assured only because God is or exists, and because he is a perfect Being.
Gist of Idea
Clear and distinct conceptions are true because a perfect God exists
Source
René Descartes (A Discourse on Method [1637], §4.38)
Book Reference
Descartes,René: 'Discourse on Method/The Meditations', ed/tr. Sutcliffe,F.E. [Penguin 1968], p.58