One of our commenters asked about whether the Lotus Sutra was considered authentic according to the Theravadin view.
To answer this from the traditional Theravadin point of view, all the Mahayana Sutras are inauthentic in the sense that they were not spoken by the Buddha. Historically, Theravada has tended to take a dim view of Mahayana, regarding it as a mere degeneration of the pure teachings.
That the Lotus Sutra and other Mahayana Sutras were not spoken by the Buddha is unanimously supported by modern scholarship. I don’t know of a single academic in the last 150 years who has argued otherwise. The basic historical background is given in Wikipedia. The upshot is that the Lotus Sutra was composed over a period of time, or in a number of stages. The oldest sources probably stem from a little before the common era, and it was finalized around 200 CE. This makes it one of the earliest Mahayana Sutras (and it is even argued that the earliest form of the sutra may not have even been Mahayana).
So there is no doubt that the Lotus Suta and other Mahayana sutras are historically late, dating from many centuries after the Buddha. When reading them as historical documents, rather than seeing them as spoken by the Buddha, we should see them as the response and articulation by Buddhists of the past to the conditions that they were in. They were addressing matters of concern for them, asking how the Dhamma is to be applied in these situations. Of course the same is true of many Theravdin texts, although in the case of the early Suttas and Vinaya there is still a core that probably stems from the Buddha himself.
Why were the Mahayana Sutras phrased as if spoken literally by the Buddha? This is a difficult question, and there is unlikely to be one answer. Partly it was just how the literary form evolved. But I suspect, given the visionary nature of many Mahayanist texts, that they often stemmed from meditation experiences; visions of the Buddha, memories of ‘teachings’ received while in samadhi. Perhaps the authors of these texts believed that the Buddha was really present to them in some sense – and this is indeed the theme of many Mahayana sutras. Or perhaps they more humbly believed that they had gained insight into the Dhamma in some direct way.