Here's a touchy subject. Modern scholarship suggests that Mahayana sutras were composed centuries after the Buddha died. The traditional Mahayana explanation is that the Buddha gave these teachings to his closest disciples to disseminate at a later point because the sutras were too advanced for followers to understand during his lifetime. Some teachers take this literally, while others view this as an attempt by early Mahayanists to legitimate the sutras they themselves were composing. Advocates of the latter theory have no quarrels with the possibility that the Buddha himself did not recite these sutras, for as far as they are concerned, the teachings represent a deep, absolute truth, so it doesn't matter who composed them.
I've been ruminating on this subject a lot lately. Did the Buddha recite the Mahayana sutras? And if he didn't, does it matter? I don't know. I suppose that as long as the Mahayana sutras don't contradict the Buddha's other teachings—which, to my knowledge, they don't; rather they expand on ideas that were already present in the earlier Canon—then it doesn't matter to me who composed them.
What really interests me is whether or not early Mahayanists believed this. In other words, when Mahayana Buddhism was spreading to Tibet, China, Korea, and eventually Japan, did these "founding fathers"—to borrow a phrase from American history—themselves question whether the Buddha himself recited the Mahayana sutras. I think the answer is a clear no. Fifteen hundred years ago, Buddhists didn't have access to the breadth of the Buddha's teachings like we do today. It's probably safe to say that the average Buddhist only heard/read several sutras in his/her entire lifetime—hence the reasons why some Mahayana schools devote themselves almost entirely to one sutra. So there would be little if any opportunities to compare the style, vision, and scope of the Pali Canon to Mahayana sutras. For these early Mahayanists—Tsongkhapa, Bodhidharma, the Fifth Patriarch—not only were the Prajnaparamita sutras the Buddha's teachings, but they may have been the only teachings familiar to them. Or at the very least, these teachings would be considered the most important ones.
This offers a viable explanation to a quandary that has fascinated and perplexed me for a long time—why Mahayana schools like Zen and Vajrayana tend to de-emphasize what other schools view as core Buddhist teachings (i.e. Mindfulness, the Four Noble Truths, and the Eightfold Path). The reason may be that their founders didn't have access to these early sutras, and instead relied almost entirely upon the Mahayana canon. This would explain why, in addition to the influences of local cultures, these traditions branched off in the directions that they did—Zen emphasizes non-duality while Vajrayana concentrates on emptiness. This would also explain why some later Mahayana masters' teachings don't easily fit with the Buddha's original suttas (I'm thinking of Dogen, who on at least one occasion argues against rebirth, and whose understanding of nirvana is drastically different than the one found in the Pali Canon).
This is a pet theory of mine. I'm no scholar, and I'm sure that greater minds than mine have explored this topic. If you've read about it, please let me know. Either way, tell me what you think.
Image borrowed from Creative Commons flickr user: nathan x. sanders