Full Idea
Since (for romantics) we are wills, and we must be free, in the Kantian sense, controllable motives count more than consequences, and the greatest virtue of all is what existentialists call 'authenticity' and what romantics called 'sincerity'.
Gist of Idea
If we are essentially free wills, authenticity and sincerity are the highest virtues
Source
Isaiah Berlin (The Roots of Romanticism [1965], Ch.6)
Book Reference
Berlin,Isaiah: 'The Roots of Romanticism' [Pimlico 2000], p.139
A Reaction
The case of the sincere or authentic Nazi shows the problems with this. However, I agree that sincerity is a key virtue, perhaps the crucial preliminary to all the other virtues. It is hard to imagine a flow of other virtues from an insincere person.