Themes of The Journey to the West include redemption, overcoming adversity, and Buddhism.
The Monkey King is the main character who has to work for redemption. His pride and arrogance led to his imprisonment by Buddha; he stays there for 500 years. When he's finally released, he isn't immediately forgiven. Instead, he has to accompany a monk sent to find and return Buddhist scriptures to China to help guide the people there. His mistakes don't color his eternal existence, though. He's able to work to earn forgiveness through the good deeds that he does. In the end, he's raised to be a Buddha himself.
Overcoming adversity is another theme in the book. Hsüan Tsang is born after his father is supposedly killed by a ferryman who then usurps his wife and job. His mother dumps him into a river and he is adopted and raised as a monk. When he finds out the truth about his parents, he reveals the crimes of the ferryman and his father is returned by the god of the river. Hsüan Tsang goes on to undertake a journey to return Buddhist scriptures to China. In order to do so, he has to face dozens of ordeals along with the help of his companions. By overcoming them through strength, wisdom, and bravery, they're able to receive their eternal reward.
Buddhism is an important theme in the book and can be seen partially as a representation of truth. Buddha is the only character able to imprison the Monkey King for his transgressions. Hsüan Tsang, the Monkey King, and their companions go on a journey to receive the scriptures that should be returned to China from India. The quest goes on for years but returning the essential truths of the Buddhist scriptures is an essential job that is given to them by deities. In the end, the Monkey King and Hsüan Tsang are taken to paradise and made Buddhas, which is their reward for completing their quest.