An undecidable proposition is one whose truth or falsity cannot be determined. A paradox is one kind of undecidable proposition. Please note that a paradox is not the same as an inconsistency. You might want to try reading the first sentence of this paragraph again and see if it makes more sense now. For this reason I will attempt to illuminate a major paradox of nonviolence theory.
Any theory is a system of belief.
A theory of nonviolent struggle is a system of belief about how people should act toward one another while attempting to change a society. A society is, among other things, a system of belief about how people should act toward one another in general. Thus any theory of nonviolent struggle is a system of belief about systems of belief and is capable of referring to itself among those social systems it discusses. This is all well and good. Self-reference alone does not constitute a paradox.