Christians are supposed to pay their tithes because God commanded Christians to give a tenth of their income in the book Numbers 18. Not paying your tithe, according to Malachi 3:8, is robbing God. Tithing is considered part of the Law of Moses.
The Old Testament book Malachi 3:8 says, "Will man rob God? Yet you are robbing me. But you say, 'How have we robbed you?' In your tithes and contributions". Christians interpret this verse to mean that they are biblically mandated to pay their tithes and face being labeled as thieves in the eyes of God if they do not. Christians also believe Malachi 3:10 means they should pay their tithes so that God can release his blessings unto them.