A sermon is a part of a Christian church/mosque ceremony in which a priest/minister gives a talk on a religious or moral subject, often based on something written in the Bible/Koran. Example: The Reverend William Cronshaw delivered/preached the sermon. Today's sermon was on the importance of compassion.= a long speech in which the minister/priest advised people gathered in the church about how they should behave in order to be better people. Usually a priest will talk about helping your neighbours, poor people, being sympathetic to the suffering of others etc.

Today's sermon was on the importance of compassion. means: sermon: speech in religion practice (like in mosque or church), or Dakwah in muslim compassion: sympathy or something like that so, Today's sermon (dakwah) was about the importance of having sympathy