"And it came to pass that many of the Lamanites did come down into the land of Zarahemla, and did declare unto the people of the Nephites the manner of their conversion, and did exhort them to faith and repentance. Yea, and many did preach with exceedingly great power and authority, unto the bringing down many of them into the depths of humility, to be the humble followers of God and the Lamb." (Heleman 6:4-5)
What was their "great power and authority"? The power of God himself; their words were rejoiced over in the heavens and the Holy Ghost testified with them.