Most of us have told the joke about the thermos. We may tell this joke about our biggest football rival, about people with certain colors of hair, or about rednecks. In the joke someone is introduced to a thermos. They are told: "It keeps hot things hot and cold things cold." And the dumb response is: "How does it know?"
Constantly I hear people speak of how they know that God wants this or that for them. A young man moves to a city and attends a church for the first time, a denomination with which he has had no association in the past. His immediate reaction is: "I knew this was where God wanted me to be." A lady switches from one religious group to another. She feels some uneasiness with her change on the part of a friend. She responds: "I think I am doing what God wants me to do." A man leaves his wife and runs off with a girlfriend to whom he later is married. When questioned about his decision, he says: "I know this is what God wants for my life so that I can be happy."
My question for all of these people is: "How can you know for sure?" How can you know if you are where God wants you to be? How can you know that you are doing what God wants you to do in order to be happy? Mankind has a long habit of transforming personal opinion into God's will. We make God over in our image. God challenges our assumptions that he thinks like we do: "You thought that I was one just like yourself" (Ps. 51:21). We assume that what we want, what makes us feel good, must be what God wants. And when our conscience disturbs us, we try to beat it back by saying: "This is what God wants for me."
The only way we can really know what God wants for us, beyond the few moral principles we can learn from reason and nature, is to study our Bible. If we can not find it in THE BOOK, then we do not have any reason to say: "I think God wants..."