Can You Talk About "True Islam"?

There is a section of Evangelii Gaudium that I'm not clear I agree with, but it's not the economic sections. Near the end, Pope Francis has some pointed things to say both about toleration for immigrating from Muslim countries and about the necessity that Muslim countries protect the safety and religious freedom of their Christian residents. However, he wraps up by saying:

Faced with disconcerting episodes of violent fundamentalism, our respect for true followers of Islam should lead us to avoid hateful generalisations, for authentic Islam and the proper reading of the Koran are opposed to every form of violence.

One hears this sort of thing every so often, but I'm not clear what it means to talk about "authentic Islam" or a "proper reading of the Koran".

It could mean, "To the extent that Islam is true, it rejects violence as a means of spreading its beliefs," and if so, I can certainly agree with that.

But what people seem to mean when talking about true Islam being a religion of peace is that somehow those Muslims who believe that their faith endorses the use of force at times to spread the faith of punish unbelievers are incorrectly interpreting Islam and that if they were better Muslims, they would reject violence.

However, it's problematic to say what is "true Islam" and what is "false Islam" -- especially given that I don't think Islam is actually true, except to the extent it happens to hold things which are also held by Catholicism (such as, say, the existence of God.)

There are some things one can say definitely are, at least, held by Muslims. For instance "there is no God but Allah and Mohammad is his prophet" would seem to be something held by Muslims, and I think that even an outsider could be confident in saying that if someone believes in no God or many gods, or if he doesn't believe that Mohammad was God's prophet, then that person is not a "good Muslim" or a "true Muslim".

When it comes to a point which is disputed among Muslims themselves, however, I'm not clear how to distinguish right from wrong interpretations. There is no central authority in Islam similar to the pope in the Catholicism, and even with Catholicism, if you're an outsider and don't believe that the Church is Christ's true Church on Earth, who is to say that the magisterium is actually "true Catholicism" and not some distortion of it. At most, it seems like one could talk about "what Catholics believe" in some sociological sense.

This isn't a problem unique to Islam. For instance, do "true Protestants" believe in predestination? I'm not clear one can answer that claim. Some Protestants believe in predestination and others don't. Who is to say who the "true Protestants" are? Unless you are Protestant and you're committed to believing a specific set of beliefs within the range of what various Protestants believe, I'm not clear how you can rule on that question.

Certainly, I think that Muslims should not endorse religious violence, and I support those who believe their religion rejects it. However, I'm not clear how we can claim one way or the other what "true Islam" says on the matter.