It being impossible to change the State-religion in Venice, Sarpi had no inducement to leave his country and to pass his life in exile among prejudiced sectarians.] Very much depends on how we define the word Protestant.

If Sarpi's known opinions regarding the worldliness of Rome, ecclesiastical abuses, and Papal supremacy, constitute a Protestant, then he certainly was one.
But if antagonism to Catholic dogma, repudiation of the Catholic Sacraments and abhorrence of monastic institutions are also necessary to the definition, then Sarpi was as certainly no Protestant.

He seems to have anticipated the position of those Christians who now are known as Old Catholics.

This appears from his vivid sympathy with the Gallican Church, and from his zealous defense of those prerogatives and privileges in which the Venetian Church resembled that of France.

We must go to his collected letters in order to penetrate his real way of thinking on the subject of reform.