The idea of an eternal Buddha is a notion popularly associated with the Mahayana scripture, the Lotus Sutra, which was written down about 500 years after Shakyamuni's preaching of it.
The notion of an eternal Buddha perhaps finds resonance with the earlier idea of eternal Dharma/Nirvana, of which the Buddha is said to be an embodiment.
The idea of an eternal Buddha is a notion popularly associated with the Mahayana scripture, the Lotus Sutra, and is also found in other Mahayana sutras.
While some variations of Buddhism believe in an impersonal eternal Buddha or creative force, in general Buddhism sees the universe as eternal and without a starting point of creation.
Some variations of Buddhism express a philosophical belief in an eternal Buddha: a representation of omnipresent enlightenment and a symbol of the true nature of the universe.
The primordial aspect that interconnects every part of the universe is the clear light of the eternal Buddha,where everything timelessly arises and dissolves.
In this realm, Tathata (Reality "as-it-is"), emptiness, dependent co-arising and the unconditioned, uncreated, perfect and eternal Buddha are one.
Moreover, in certain Mahayana sutras, the Buddha, Dharma and Sangha are viewed essentially as One: all three are seen as the eternal Buddha himself.
Basing itself on the Pali suttas and meditative experience, it teaches that the Dhammakaya is the eternal Buddha within all beings.
Guang Xing speaks of how the Mahayanists of the Nirvana Sutra understand the mahaparinirvana to be the liberated Self of the eternal Buddha: