Jataka texts say that Sakyamuni Buddha accumulated a good amount of merit through his acts such as making offerings as a layman in his previous life. And it is understood that as a reward, he became a buddha in this life. But under the theory of reincarnation, a layman who accumulates merits, in general, would be born as a man or a deity in his next life, and he could not become a buddha liberated from reincarnation. To solve this problem, monks (bhiksus) thought that the Buddha had bodhi as his essence (sattva), namely, he was a “bodhisattva.” Therefore we should say that the original meaning of “bodhisattva” was “the one who has bodhi as his essence (sattva).”
Later, some monks who were worried about the trends of the age that the “True Doctrine (saddharma) is dying out” insisted that the purpose of Buddhism was not to become an arhat but to become a buddha, and they proceeded towards Awakening (bodhau sampratisthati). Then they called themselves “sentient beings (sattva) who seek for bodhi” i. e., bodhisattva, and with this consciousness they entered into the way of practice of monks. And this is the beginning of Mahayana Buddhism.