What is the difference between a sermon and a speech? (Explain)

As nouns the difference between speech and sermon

is that speech is (label) the faculty of uttering articulate sounds or words; the ability to speak or to use vocalizations to communicate while sermon is religious discourse; a written or spoken address on a religious or moral matter.

A sermon is a talk on a religious or moral subject, especially one given during a church service and based on a passage from the Bible. (Jesus talks, e. g)

A speech is human vocal communication using language. (It could be similar towards Martin Luther King and his speech which changed the world.)