Full Idea
If we exclude the idea of God, it is impossible to have an exact idea of virtue.
Gist of Idea
We can't exactly conceive virtue without the idea of God
Source
Joseph Joubert (Notebooks [1800], 1808)
Book Reference
Joubert,Joseph: 'Notebooks', ed/tr. Auster,Paul [nyrb 2005], p.130
A Reaction
I suspect that an 'exact' idea is impossible even with an idea of God. This is an interesting defence of the importance of God in moral thinking, but it only requires the concept of a supreme being, and not belief.