Discuss similarities and differences in belief and practice between Christianity and Islam.
Discuss: Refers to examine and consider (this does not mean describe). So, the essay should discuss various viewpoints, considerations, or questions (for and against, positive or negative) based on the available evidence in relation to the subject matter/issue.
Compare and contrast: Look for differences and similarities. So, you need to identify and discuss the differences and similarities of the essay topic you have chosen.