Helaman 5:19

therefore they did speak unto the great astonishment of the Lamanites, to the convincing them, insomuch that there were eight thousand of the Lamanites which were in the land of Zarahemla and round about, baptized unto repentance, and were convinced of the wickedness of the traditions of their fathers.

So they spoke and amazed the Lamanites. They convinced them. Eight thousand Lamanites in and around Zarahemla were baptized to show they stopped sinning. They were convinced that their fathers’ traditions were wrong.

They spoke with such conviction that the Lamanites were completely amazed. Eight thousand Lamanites in and around Zarahemla were baptized and turned from the wicked traditions they’d inherited from their ancestors.