I think the problem you're encountering is that in Mythic Europe, a great majority of people the characters are likely to encounter believe that statement to be the case. So the setting, at least, should support that idea. I believe that means that the divine realm should be able to have a meaningful effect on the game world, at least as much influence as the other realms, so that these masses can point to it and say, look, the divine is powerful, therefore it is my favorite realm, and since it is my favorite realm, it is the best realm. Other characters don't have to agree with them, but at least in the setting as written this should be an easy and justifiable leap to make.
I also think it's important to separate the rules from the setting. There shouldn't be anything in the rules that says that worshiping the divine is the only correct way to worship, or that those who throw in with faeries or magical beings are wrong or deluded. (Or the infernal, for that matter.) The rules should confirm that those characters associated with the realm receive demonstrations of the realm's power. If you do worship the divine, the game rules give your character appropriate divine rewards. If you worship faeries, the rules give the character other appropriate faerie rewards. All this is true, yes?
I believe the authors of the Realms of Power books tried very hard to come up with thematically appropriate manifestations of each realm's power, and for the most part were very successful. There are admittedly a few rules systems in The Divine book that are very Christianity-centered and don't translate very well to other faiths (saintly intercession and relics immediately come to mind), but I don't think that they prove that Christianity is right so much as suggest that the divine is terribly schizophrenic. Divine tempers are organized against the seven deadly sins, which is again very Christian, but I generally think of them as more of a framework of the sort of environments that divinely-focused characters can foster. Perhaps there are other tempers associated with other divine religions?
Actually, one thing I can think of that might mechanically support the "divine realm is paramount" idea is the manifestation of sin. There are holy powers that can recognize the stain of sin on an individual. The fact that some characters can see this might suggest that the actions taken by the targeted character were "wrong" or "right" according to the divine character's religion. But if we see this effect as instead probing the target's memories, looking for particular actions that the divine character believes oppose her faith's tenets, not seeing signs of objectively "wrong" behavior, then the value judgment on these actions is kept in character where it belongs.
Can you come up with some more examples of rules that you feel enforce the "divine realm is right" idea?