 under the power of God's Holy Spirit, because Romans 10, 17 says, Faith cometh by hearing, and hearing by the word of God. So God uses his child to give his testimony, and if he's called that individual to preach, all of his preaching is based upon the word of God and what God has done to that individual. The agent is the Holy Spirit. The means is the word of God, and the instrument is his messengers.