Why do you think God uses His people to spread the Gospel rather than another method?
Peter witnessed to Cornelius by telling him what he saw in Christ's life, death, and resurrection. Cornelius and friends believed the message and then Peter witnessed them receive the Spirit and be baptized. And then as Peter is being questioned by the Jerusalem church leaders, he spoke of all that he witnessed the Lord do with the Gentiles. The church leaders believed the message and glorified God.
God calls all of His followers to be witnesses of Him, His work, and His salvation (Matthew 28:18-20, Acts 1:8). Today, as we witness with our words and our actions, we trust that God will use it to advance His Kingdom. What a honor it is that God wants to use our words and our actions for His purposes.