Opened his mouth - See Matthew 5:2.
At the same scripture - Taking this as a “text” to be illustrated.
Preached unto him Jesus - Showed him that Jesus of Nazareth exactly corresponded to the description of the prophet, and that therefore he referred to the Messiah, and that the Messiah was Jesus of Nazareth. How far Philip detailed the circumstances of the life and death of Christ is unknown. What follows shows also that he stated the design of baptism, and the duty of being baptized.