Are the Jafari, Zaydi and Ibadi schools part of Islam?

Fatwa, posted 4.22.2010, from Syria, in:
Religious Authority: 
Ahmad Kuftaro
Fatwa Question or Essay Title: 
Are the Jafari, Zaydi and Ibadi schools part of Islam?
Websites and Institutions: 
Amman Message, (Jordan)

[The following is excerpted from a longer piece; see record image for original text.]

We should not be surprised at the differences between scholars of Islamic jurisprudence regarding tangental matters, since the religion is one, the law is one, the truth is one and cannot be quantified. Similarly, the source is one: divine revelation.

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