From time to time one sees statements, most typically from Eastern Orthodox respondents, as to what "Protestantism" holds. Now theoretically, I'm a Protestant, being an Episcopalian and all that. Well, you can find Anglicans who say that Anglicans aren't Protestant, and you can find others who'll claim that some Anglicans aren't Protestant. But at any rate, what's the "-ism" in "Protestantism"?
It's easy enough to find a common historical origin to protestant churches: their separation from the Roman Catholic Church. This implies but two "doctrines": that the claims of the RC church to infallible teaching are false, and that there is grace outside of the RC church. Other doctrines? Well, there is hardly any topic which does not elicit radical disagreement somewhere among some "protestants".
It's not hard to see the deeper meaning of statements as to what Protestants believe. These statements are about differentiation. Converts are particularly susceptible to making them because converts need differentiation badly; conversion, after all, involves ceasing to be of one's former group. But the urge to differentiate is at best dubious, because it tempts one to exaggerated generalizations, to the point of absurdity. There is no system in "Protestantism, not when one tries to put the Watchtower Bible and Tract Society in the same bin with Wheaton College, Bob Jones University, and Episcopal Divinity School.