Why there is no “becoming a Buddha”
Kṣitigarbha made a vow unlike any other bodhisattva's: not to attain buddhahood until the hells are emptied and every being has been carried across. In the sutra the words are: “For countless kalpas to come, I will devise every means to liberate the beings of the six paths who suffer for their offences — and only then will I myself attain buddhahood.”
The famous four-line summary — “Until hell is empty, I will not become a Buddha” — is a later Chinese condensation, not a line from the sutra. It is accurate as a summary, and it is why he is still depicted as a bodhisattva.
Four lives, four vows
The sutra tells of four occasions across past kalpas.
As the son of an elder, in the time of the Buddha Lion's Roar, he saw the Buddha's radiance and vowed to work for the liberation of all beings before attaining Buddhahood himself.
As a Brahmin woman, in the time of the Buddha Flower of Awakening, his mother had fallen into a lower realm. The daughter sold her house, made great offerings, and sat reciting for a day and a night. She was shown a sea of suffering, and told that her mother had already been reborn in a heaven through the power of those offerings. She then vowed to liberate beings for kalpas to come.
As the daughter Guangmu, in the time of the Buddha Pure Lotus Eye, his mother had killed many living things and fallen into suffering. The daughter made images, made offerings, and vowed that she would not attain awakening until every being in the hells and the three lower paths had been saved.
As one of two kings, in the time of the Buddha All-Wisdom, one king vowed to become a Buddha quickly and then save beings; the other vowed to save beings first and become a Buddha last. The second king is Kṣitigarbha.
Mount Jiuhua
The Chinese home of Kṣitigarbha is Mount Jiuhua in Anhui. A Korean monk, Kim Gyo-gak, crossed the sea in the Tang dynasty and lived there for decades; he died in 794 and his body was said not to decay. Later generations took him for an emanation of the bodhisattva and called him “Kim Kṣitigarbha”. This is history and devotion, not prophecy.
Two ways of reading it
The Sutra of the Divination of Good and Evil Karma says that he long ago fulfilled the path and is a Buddha in his own right, appearing as a bodhisattva only by the power of his vow. This does not contradict the vow: one statement is about his origin, the other about how he shows himself.
What the sutra is really about is not miracle-working. It is about someone who refused to leave first.