Thanks for the feedback, kaspergm.
Yeah, that's one of the beliefs I'm having a little more difficulty with over the other ones, which I think are pretty stable.
Here's the latest, I've highlighted the ones I'm still working on:
God of Knowledge is the trickiest one, since science representation comes early with the Library, and then much later with the University -- nothing really in between, except for the national wonder. With production or culture, along with the initial city bonus for having the population, I can give a bonus to a Late Classical/Early Medieval building to buff the belief a little later one. With faith, it has both an early building and a later building, so I can easily give those a bonus that's spread out more. (Also, comparing this faith one with the production/culture ones, I think having to build a shrine versus growing to a population of 4 is easier, so the faith one has a 6 pop requirement instead.)
I have God of Knowledge at a 6 pop requirement because there were no science flavored buildings to give a bonus to within a reasonable amount of time -- late Medieval just seems too late. I thought about giving the university a 2 science bonus to make up for it (alongside the library one), but in the end, I think it could be too much.
Another possibility was to tie some type of larger later bonus to the National College, since it's obtained within a good time frame. But, at the same time, I want to give wide empires strong beliefs if they work towards them, tall to average empires can just take advantage of the resource beliefs and others, since they'd probably offer more total bonuses over any bonuses-per-city to their limited number.
I don't believe the new version in the form of God of Knowledge is better than the default Messenger of the Gods, though. With the default one, all you need to do is connect a city, even a freshly settled one, and you receive an instant +2 science. With the new one, even if you rush buy the library needed, you still need to wait until you hit a population of 6. This also prevents, or at least slows down, the abuse that exists with the default one and rapidly expanding, since you'll need to make for 1.5 luxuries worth of happiness for each science bonus you want in your cities early on (along with the initial unhappiness from city count -- but, both beliefs deal with that).
This is all just theorycrafting and still needs testing, of course.
Another thing I was thinking about was maybe some kind of small city-state boost to Goddess of Love instead of the extra culture. When I think of "love", I don't only think of affection, but also of peace. So maybe a city-state boost would be nice (and different) alongside the 1 happiness for each city at pop 4+ when not at war. Just a thought, as I just recently noticed an used belief ability in the XML, involving city-state friendship, that I want to test out. EDIT: Actually, I just searched and found out what CityStateFriendshipModifier does. It basically has an Alexander effect. Hmm...