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 order to illustrate this point to Belldandy and attain her help, he shows her the Judgement Gate recording where Morgan and her lover were forced apart forever. However, Heaven caught onto Celestine's actions and attempted to restrain him, but Belldandy, confused and saddened, lashed out her powers and killed all that attempted to harm her mentor. However, both she and Celestine were restrained.
Celestine was then stripped of his powers by transferring his consciousness into a stone mask tablet and sent to be incarcerated in the Luna Dungeon on the moon. Eventually, Morgan and he resonate and Celestine uses Morgan's body in order to be able to interact with the world around him.
He makes contact with Belldandy again, and infects her with a virus that erases her memories after she met Keiichi as well as something that infiltrated the Heavenly supercomputer Yggdrasil; it was also a virus that was synchronized with Belldandy's emotional waves. His plan was to create a situation where Belldandy was under a lot of emotional stress in order to power up the virus, and be able to disable most of Yggdrassil while dragging it down to Earth and cutting it. Cutting Yggdrasil would enable the energy of creation to flow unrestrained upon the world, causing it to fast-forward in time.
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.