Internal controls help to avoid the compounding of errors and misunderstandings over time.
Let's remind ourselves that no amount of checks within a system will catch everything. If, unhappily, your church hires a skilled bookkeeper whose intent is to fortify his or her income in an unauthorized way, there is little you will be able to do to detect this until after a large amount of money is gone. The same procedures will make it considerably more difficult to intentionally misuse funds or to steal. The first rule is to not ask one person to do all the work. This seems pretty clear, yet is easily overlooked.