Dialectic yields strong presumption about all or most what demonstrative proofs yield certainty about, and rhetoric persuades about of what is not such as to be proven by demonstration or looked into by dialectic. Moreover, virtuous religion is not only for philosophers or only for someone of such a station as to understand what is spoken about only in a philosophical manner. Rather, most people who are taught the opinions of religion and instructed in them and brought to accept its actions are not of such a station -- and that is either due to nature or because they are occupied with other things. Yet they are not people who fail to understand generally accepted or persuasive things. For that reason, both dialectic and rhetoric are of major value for verifying the opinions of religion for the citizens and for defending, supporting, and establishing those opinions in their souls, as well as for defending those opinions when someone appears who desires to deceive the followers of the religion by means of argument, lead them into error, and content against the religion. Al-Farabi "Book of Religion" Trans Charles Butterworth, (7)
In much of Al-Farabi's writings, one finds a sharp split between the elite few and vulgar many and an accompanying fondness for wise leadership. This wise leadership ensures that within an extensive and hierarchically ordered division of labor everybody pursues virtue/happiness, and, -- this is the more egalitarian element in his thought -- attains (some) happiness. In addition, his view of true religion is Platonic in that for Al-Farabi it tends to mean a second-best way at obtaining a path toward the (eternal and invisible) truth by way of corporal images or signs. Such a religion can involve rituals and practices, including ones we would not ordinarily call 'religious,' but that are closer to civil religion and other sources of meaning. The wise leader can reveal true religion then to the masses.
But in the first sentence of the Book of Religion, Al-Farabi offers a more functional definition of religion, which "is opinions and actions, determined and restricted with stipulations and prescribed for a community by their legislator [literally, "first ruler"--ES] who seeks to obtain through their practicing it a specific purpose with to respect to them or by means of them." (1) The ends of the legislator may be beneficial or virtuous or not (etc.). This definition exhibits a proto-Hobbesian understanding of religion within a political community. (Al-Farabi still prefers virtuous religion and virtuous leadership.) Attaining the specific purposes by way of legislating a religion requires not just knowledge of universals, but also contextual knowledge (8). And, as it happens, Al-Farabi allows that successive virtuous legislators, who introduce true religions into society, can innovate if circumstances require it, while still making available a path toward truth for their citizens.
I read the final lines of the quoted passage as a defense -- to flirt with anachronism -- as a call for a modest public enlightenment -- the promotion of the widespread diffusion of rhetoric and dialectic -- in the service of true religion and the unmasking of false religions. That is, Al-Farabi thinks the citizenry should be equipped with the skills to detect argumentative fallacies and rhetorical sleighs of hand. Admittedly, Al-Farabi is not calling for the teaching of philosophy & natural philosophy, and so his proposal falls short of a plan for full public enlightenment.
Perhaps, Al-Farabi is merely limiting the knowledge of dialectic and rhetoric to would be legislators (I regret not being able to read the original)--then the quoted passage would be advocating the art of persuasion to (would be) rulers (and their assistants). But even among them the primary utility of this art is to prevent the spread of false opinions (and thereby false religion) by way of argument among the population. And even this more modest proposal still takes seriously the capacity of ordinary citizens to engage in not just valid, but even sound argument.