[It] may be possible for the religions of virtuous nations and virtuous cities to differ even of they all pursue the very same happiness. For religion is a sketch of these things or of their images in the soul. Since it is difficult for the public to understand these things in themselves and the way they exist, instructing them about these things sought by other ways--and those the ways of representation. So these things are represented to each group or nation by things of which they are more cognizant. And it may be possible that what one of them is more cognizant of is not what another is more cognizant of.
Most people who pursue happiness pursue what is imagined, not what they form a concept of. Similarly, the principles such as to be accepted, imitated, extolled, and exalted are accepted by most people as they imagine them, not as they form a concept of them. Those who pursue happiness as they form a concept of it and accept the principles as they form a concept of them are the wise, whereas those in whose souls these things are found as they are imagined and who accept them and pursue them as thought they are like that are the faithful. Al-Farabi, Political Religime, 90.
Carlos Fraenkel nicely points out that religion is a kind of imitation of philosophy in Al-Farabi. While truth is one, the happy paths toward it are diverse. Given that, for Al-Farabi, humans are differentiated not just by innate talents (his elitism is unmistakable) and by diversity of language, but also by geographically and institutionally produced differences, it follows that many kinds of religions can locally be set up (by inspired prophets, or inspired philosophers-legislators) that guide people on their path toward happiness and truth. So, while Al-Farabi undoubtedly does not think that all religious practices guide people toward truth and happiness, Al-Farabi is a religious pluralist about those that do.
It also makes him a kind of pluralist about the religion of a given political community. By this I do not mean (i) that he thinks that there should be different official religions within a city/state (although it is possible that he allows for that in a very large confederation/empire of states); but rather, first, (ii) that within any such local religion diverse paths are possible for different kinds of citizens (among the faithful and the elite few). In addition, I mean by this that (iii) what counts as 'religion' (milla) is not just practices about divinities (or God). His definition of religion is much broader than that: any practice that can orient a believer toward the path toward the one truth (the rule of reason) is religious: this includes not just worship of God, but, in a well-ordered state, the practices of politics, public rituals, cultivation and education.
That is to say, Al-Farabi's notion of religion includes what we would call 'civic religion' not just in Spinoza's and Rousseau's sense of the shared religious beliefs (about the divine) of a community, but also in the more modern sense (also present in Spinoza and Rousseau) of the practices of a community that bind it together as a unity (oriented, indirectly, toward the Good/God/Reason).