The Holy Spirit
The prophecy in Psalm 40:7, points the way to the fulfillment of all prophecy just as Jesus taught in the Gospels of His life, death, and resurrection. If we carefully and intently look into all Old Testament prophecies, we can, just with the Old testament scriptures lead many to repentance, confession, and Immersion. There-by allowing the Holy Spirit to convict and Jesus to save.
God is good. Love you all.