First Samuel 16 begins the history concerning David as one prepared by God as a man according to the heart of God. Chapters sixteen and seventeen show us how David was chosen, trained, anointed, tested, and approved by God.
Chapter fifteen ends with a miserable situation. God had abandoned Saul, rejecting him from being king over Israel. Then in the first verse of chapter sixteen, God sent Samuel to contact a young boy, who was perhaps only about fifteen years old. This shows that while Saul was abusing the God-given kingship, God knew the whole situation and was doing something marvelous to prepare the right person. God went secretly to David, the great grandson of Boaz and Ruth (Ruth 4:21-22).
God gave Saul the kingship for the purpose of building up His own kingdom. Saul should have been God's acting king, reigning over the people for God. Yet he behaved as a king absolutely for himself. He usurped and even abused the gifts given to him by God—the kingdom, the people, and the kingship—in order to build up his own monarchy. But while Saul was openly abusing, God was secretly preparing. Eventually, God's preparation was completed in David, the youngest of Jesse's sons. David was the eighth son, and in the Bible the number eight signifies resurrection. In the scriptural sense, David was one in resurrection; hence, he was the one whom God could use.
David was prepared by God to be a man according to the heart of God. Otherwise he could not have been such a man. In a very real sense, he was created by God to be such a man. However, he still needed to pass through the process of being chosen, trained, anointed, tested, and approved. To be chosen is wonderful, but to be trained by God is to go through suffering. To be anointed also is very good, but after being anointed we must be tested. Second Samuel 5:4 tells us that David was thirty years old when he began to reign. After David, a boy of about fifteen, was anointed by Samuel, he was tested for about fifteen years. In particular, he was bothered and troubled by Saul. Eventually, David passed the test and was approved by God.