He [the Buddha] said to the four kinds of devotees: “Devadatta1 will become a Buddha after innumerable kalpas [eons]”.
–The Lotus Sutra, Chapter 12, Murano Translation
The early Buddhist community made a clear distinction between Shakyamuni, the Buddha, who was a fully and self-awakened being (i.e. no one was there to teach him the Dharma) and other disciples who attained awakening through the teachings of the Buddha. These disciples were known as arhats or “noble ones”. Although both the Buddha and the Arhats experienced the same awakening, the Buddha had additional talents and capacities that allowed him to lead and teach the community.
The ancient Jataka Tales contained many stories about the past lives of Shakyamuni Buddha and described the many heroic deeds as a bodhisattva he committed in his endless pursuit of awakening.
In light of this, it was thought that such an arduous path across so many lives and so many heroic deeds was all but impossible for others, and that one should settle for liberating oneself as an Arhat.
However, by the time of Mahayana Buddhism, it was felt that everyone was capable of becoming a Buddha, and so the Arhat stage was simply an important milestone on the road to becoming a Buddha. After liberating oneself, one would turn outward and seek the same awakening as a Buddha, but also help and liberate others along the way. Thus an Arhat turned into a Bodhisattva.
In any case, the implication was that all beings were capable of becoming a fully awakening Buddha, even Devadatta who actively betrayed and sabotaged the Buddha, and is said to dwell in the worst of the hell realms for many kalpas (eons). It is simply a question of time and by what means.
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1 Devadatta was a cousin of the Buddha, and formerly a disciple until he sought to split the monastic community and set himself up as a rival teacher. Further, he attempted to kill the Buddha on three occasions, but failed. Ultimately, he regretted his actions, but as a result of his terrible deeds, the earth opened up and swallowed him, condemning him to the lowest level of Hell for many eons. The point of the passage above is that even Devadatta will ultimately find redemption and become a Buddha someday.