He meditated for five yugas (cosmic eras) as a bodhisattva. Thus having acquired much merit, he was able to create Sukhavati, a pure land where humans could be reborn and live without suffering.
During his time in meditation, Amitabha had made 48 vows to save all sentient beings from samsara.
Of these vows, in the 18th vow he says that any sentient being that recites his name even ten times will be reborn in his pure land.