It has been shown conclusively that in the earliest sutras Buddha is shown as having attained nirvana in this lifetime, and did not lose it during the decades before his death. Hundreds of years later, in Normative Buddhism, the early picture of Buddha’s enlightenment as nirvana had become increasingly modified, to the point that many came to consider it impossible to attain nirvana in one lifetime. Nevertheless, this must not mislead us into thinking that such was the view of the Buddha’s followers in his lifetime, or soon after his death. It is logically necessary for the Buddha to have achieved nirvana and for his followers to have believed that they could do the same thing if they imitated him, in order for such later ideas to have developed in reaction to it. If the Buddha had not achieved his remarkable, heroic breakthrough, there would have been no Buddhism.
Beckwith, Christopher I.. Greek Buddha: Pyrrho’s Encounter with Early Buddhism in Central Asia (pp. 42-43). Princeton University Press. Kindle Edition.
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In early Buddhism, it is also true that no one thought of the Buddha as a god. The Buddha himself, even in ‘Normative Buddhist’ texts, claims that he is ‘just a man’ and that anyone can achieve nirvana if they do the work (karma). It would probably be a good thing for Buddhists today to emphasize these points; and by so doing, remind Buddhism of its deepest roots — Nirvana is a real state that anyone can achieve if they do the work; and all dharmas (things) are characterized by the Three Signs, or Trilakṣaṇa: “All dharmas are anitya ‘impermanent’…. All dharmas are duḥkha ‘unsatisfactory, imperfect, unstable’…. All dharmas are anātman ‘without an innate self-identity’.” ABN
American Buddhist Net 