Full Idea
Since God does not wish to deceive me, he assuredly has not given me a faculty of judgement with which I could never make a mistake, when I use it properly.
Gist of Idea
Since God does not wish to deceive me, my judgement won't make errors if I use it properly
Source
René Descartes (Meditations [1641], §4.54)
Book Reference
Descartes,René: 'Discourse on Method/The Meditations', ed/tr. Sutcliffe,F.E. [Penguin 1968], p.133