There are four:
“That was the first criterion I had for a wife—female. The second was: She treasures Jesus Christ. The third was that she like me. The fourth was that she be willing to go wherever God calls us. (Remember the conversation on the couch in your apartment that first summer when I named the hardest places I could think of, and you and said, Yes? I had not even asked you to marry me yet.)”
From a letter to his wife Noel on her most recent birthday.