1 Samuel 15:17
ESV - 17 And Samuel said, "Though you are little in your own eyes, are you not the head of the tribes of Israel? The Lord anointed you king over Israel.
For follow-up discussion and general commentary on the topic. Comments are sorted chronologically.
Here's what I think: Israel to this day practices an unauthorized form of worship in spite of the rejection signified by the death of Jesus and the destruction of the temple (a necessary part required by God) in His relationship with them. Saul's continued "rulership" foreshadows this as the Church now represents God's anointed. The establishment of the Kingdom of God in fullness on the earth, as prophesied by Isaiah will come at Jesus's return - the fulfillment of the eternal reign of the Son of David. Saul represents a legal relationship; David a grace relationship.