Homosexuality
An obvious theme in Maurice is that of homosexuality. The whole story is about Maurice as he discovers, expresses, and learns about his homosexuality and the different forms it can have. He goes from experimenting to platonic to raw as he meets new people with different views in the novel.
Mysticism
Another theme in Maurice is that of mysticism. There is a suggestion of something influencing Maurice that is beyond our human senses. It is portrayed in different ways, like in his childhood dream, and how he is able to sense Alec outside his window despite him not knowing.
Self-Acceptance
The third theme is self-acceptance. The different characters show different ways of accepting their homosexuality. Maurice discovers, explores, and develops an inner acceptance. This is not the case for Clive, who denies his love for Maurice and ends up leaving him.