Celestine was a high-ranking god in Heaven, and was on one of Heaven's council seats. He served as Belldandy's mentor for most of her training, ever since she was a young girl.
Eventually, Celestine saw too much suffering in the third dimension. He knew that Belldandy would eventually become an agent for the Goddess Relief Office, and did not wish to expose her to such a cruel world. His plan was to fast forward time on Earth so that the world would evolve past hurting itself.
In the end, Celestine failed. He failed to realize that although this method would cause things to age, it wouldn't cause things to mature. Belldandy was happy on Earth, living, learning, and loving. He left peacefully after he had realized this.