It was a great and a surprising victory, given to Israel by the Lord, contrary to the expectation of most of Israel. However, it was exactly what David had by faith told Goliath would happen.
Why did Saul not recognise David? ‘When Saul saw David go forth against the Philistine, he said unto Abner, the captain of the host, Abner, whose son is this youth?’ Surely Saul would have recognised him! There are two possible explanations for this question. It is possible that some time since Saul last saw David, and the young man had changed his appearance since then. But the question is not ‘Who is this young man?’ but ‘Whose son is this youth?’ The passage does not necessarily imply that he didn't recognise him at all. All it says is that he asked whose son he was. He knew who he was. He had spoken to him before the battle. His words been reported to him. He had been his minstrel; he had been his favourite; he had been his armour bearer. Of course he recognised him. Saul had after all said that whoever felled this Philistine would have his daughter to wife, and that his father's house would be free in Israel (verse 25). He knew David, but he didn’t know anything about his family. He didn't know the details, the legal details, and so probably all he is doing is asking Abner to find out about the family. That is the traditional way of interpreting the passage among Reformed commentators.