This is the only time in Scripture that the Spirit of the Lord is said to have left someone.1 In this context, it relates to empowerment and not regeneration. No longer would the Spirit empower Saul in a special way to help him rule as king.2
The Spirit's departure from Saul doesn’t have reference to his faith. It doesn’t tell us about his heart. Instead, it is a confirmation that he has been rejected by the LORD from the office of king. This departure of the Spirit from Saul is the exact opposite of what happened to David.
14 Now the Spirit of the LORD departed from Saul, and a harmful spirit from the LORD tormented him.