It says so in the Bible!
“It is wrong for Christians to drink/be in a same-sex relationship/get divorced/use Facebook/drive a Prius/issue du jour—it says so in the Bible!”
“Where does it say that in the Bible?”
It is surprising how often people can’t answer that.
If you are going to use the Bible as a basis for your beliefs (as I believe all Christians should) and you are going to share your beliefs with other people, then you should be able to locate the relevant passages. BibleGateway is a useful resource for searching the Bible in various translations.