What was the Atreides worldview?

Question

Appendix Ii of Dune notes an upsurge of galactic interest in the Zensunni belief system after the triumph of Paul and the Fremen. Which raises the question, what was he before he became a Zensunni? It clearly wasn't widely held or well known.

The first four novels make much of the difference between the Atreides' worldview and that of the Harkonnen, Fremen, etc. At one point in Children of Dune, Leto II even calls Paul out on having flawed 'Atreides' beliefs compared to his superior ones, fully identifying himself as a Fremen rather than an Atreides.

But I haven't been able to work out what exactly the Atreides believed, beyond a general sense of paternal noblesse oblige?

Answer 1

Paul Muad'Dib and the Atreides family in general seem to have no major religious affiliations. Paul has extensively studied (under the tutelage of Thufir Hawat) the text of the Orange Catholic Bible, a book which is described in the text as...

...contain[ing] elements of most ancient religions, including the Maometh Saari, Mahayana Christianity, Zensunni Catholicism and Buddislamic traditions

But there is no evidence that he has any belief in a personal God, rather his belief-system is quite adequately described in the text of the O.C Bible itself as being one of the agnostic rulers who should respect religion, even if they don't themselves believe in it;

“Men, finding no answers to the sunnan [the ten thousand religious questions from the Shari-ah] now apply their own reasoning. All men seek to be enlightened. Religion is but the most ancient and honorable way in which men have striven to make sense out of God’s universe. Scientists seek the lawfulness of events. It is the task of Religion to fit man into this lawfulness.”

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You may wish to note that Paul has no problems with using religion for his own ends (his mother was after all a Bene Gesserit, a group whose main trade is designing religions to suit their needs), nor does he have any issue with impersonating a deity. He also has no problem whatsover with acts of blasphemy, all of which points to someone who has grown up without any major religious ideals:

“The Fremen have the word of Muad‘Dib,” Paul said. “There will be flowing water here open to the sky and green oases rich with good things. But we have the spice to think of, too. Thus, there will always be desert on Arrakis … and fierce winds, and trials to toughen a man. We Fremen have a saying: ‘God created Arrakis to train the faithful.’ One cannot go against the word of God.”

The old Truthsayer, the Reverend Mother Gaius Helen Mohiam, had her own view of the hidden meaning in Paul’s words now. She glimpsed the jihad and said: “You cannot loose these people upon the universe!”

Answer 2

It is presumable that the Atreides, along with the rest of the ruling class, were agnostic adherents of the Orange Catholic Bible. You mentioned Appendix II in your question, and most of my answer draws from there.

Religion, like the other main forces in the Imperium, is also in a state of stagnation. The O.C. Bible was specifically made to defang religion in the wake of the Butlerian Jihad:

"We are here to remove a primary weapon from the hands of disputant religions. That weapon -- the claim to possession of the one and only revelation."

The Appendix lays out the dominant religious beliefs at the time of the Imperium. It doesn't rank them or even truly define what "dominant" means, but there's the O.C. Bible and then there's several "ancient teachings" found on specific planets. We must assume this means the O.C. Bible is followed everywhere besides those specific planets. The other two "religious beliefs" are the Bene Gesserit (who deny they are a religious order) and then the agnostic rulers:

The agnostic ruling class (including the Guild) for whom religion was a kind of puppet show to amuse the populace and keep it docile, and who believed essentially that all phenomena -- even religious phenomena -- could be reduced to mechanical explanations

The Appendix goes on to later specifically state the opening of the O.C. Bible is an "obvious appeal to the agnostic rulers".

As a Great House, we have no reason to believe the Atreides were not followers of the O.C. Bible. In the Imperium, it's the default religion. Even if you don't buy it, you need to at least pretend to in order to rule.

As for Leto calling out Paul's flawed beliefs, that's a bit more complex. The series makes it clear that true religious belief is disruptive and powerful. The primary religion of the Imperium was designed to prevent more jihad. The Missionaria Protectiva infiltrated and maintained the pockets of non-O.C. religions, so the Bene Gesserit could use them when necessary.

Paul ends up using the Missionaria Protectiva to make himself messiah. As cited already in the answers, Paul tells the Reverend Mother about the Fremen saying, "God made Arrakis to train the faithful", and she grasps the horror of that statement - true believers finally unleashed into the universe.

But Paul is not a true believer himself. He knows he exploited their messianic beliefs, he does not want the jihad that follows, and ultimately ends up preaching heresy against his own religion. Leto is a true believer - he knows himself to be the messiah and commits himself to the terrible purpose.