Complex sentence: In order to live for all eternity and be presented in front of Osiris, the body of the decease had to be preserved by mummification, so that the soul could reunite with it, and take pleasure in the Afterlife.
Split into simpler sentences: In order to live for all eternity and be presented in front of Osiris, the body of the decease must be a mummy. The Egyptians mummified their dead to preserve the body, so that the soul could combine again with it, and take pleasure in the Afterlife.