Our teacher, Sakyamuni Buddha, is one among the thousand Buddhas of this aeon. These Buddhas were not Buddhas from the beginning, but were once sentient beings like ourselves. How they came to be Buddhas is this.

Of body and mind, mind is predominant, for body and speech are under the influence of the mind. Afflictions such as desire do not contaminate the nature of the mind, for the nature of the mind is pure, uncontaminated by any taint. Afflictions are peripheral factors of a mind, and through gradually transforming all types of defects, such as these afflictions, the adventitious taints can be completely removed. This state of complete purification is Buddhahood; therefore, Buddhists do not assert that there is any Buddha who has been enlightened from the beginning. — Dalai Lama