The passages of Temple of Dawn where he sits the reader down and explains the Yuishiki school for five chapters straight are, I think, firstly reflective of a personal project of research Mishima embarked upon when writing - he wasn't a Buddhist himself but nevertheless felt he had to accurately represent the kind of philosophical interest taken by Honda at that point in the story - and secondly part of a kind of overarching structural contrast set up between Buddhism and Shinto over the tetralogy as a whole. The passages detailing Honda's research into Buddhism are, if you will, a mirror image of the passages in Runaway Horses which reproduce Isao's pamphlet about the League of the Divine Wind. In very general terms you can (cautiously) say that Mishima develops a polarity. Shinto is associated with youth, vitality, purity, spirit, which at its extreme flips over into the suicidal 'pure act' realised in Isao's suicide; meanwhile Buddhism is associated with old age, sickness, flesh (esp. the desires of the flesh), everything that the aging Honda comes to embody. Yet Mishima remains to the end ambiguous: the work is never clearly a tract in favour of one or opposed to the other. Rather than according to a contrast of actual religions I would hazard the polarity responds to a polarity Mishima found in himself, his own contradictory tendencies. In 1970 he was 45 and already thought himself an old man; he foresaw a future of gentle and mildly pleasurable decay, a withering of spirit which would trickle downhill to arrive ultimately at a place much like where Honda does at the very end, the garden at Gesshu-ji: "a place where there were no memories, where there was nothing." And he chose - the alternative.
- - - Updated - - -
Anyway now that you've read that I recommend checking out his plays. An excellent set of translations were published a while ago by Columbia University; look for the book with the bright pink cover bearing the words "My Friend Hitler: And Other Plays of Yukio Mishima"