"He comes back and he doesn't know everything, he doesn't know the doctrine super well. But he has a powerful testimony that he can bear and he can share with people. A little while later he and some of the brethren in Salt Lake decide to go on a carriage ride up the eastern bench of the mountains. They decide to pull the curtains on the carriage so that they can see out the window and see the view. And when that happens, Martin sees the valley lying before him, he sees everything that's been built up, and all the saints and how large of a population is out there, and he is just stunned by it. Martin's been spending all his time in this little podunk town, Kirtland, where there's hardly anyone who will take him seriously and listen to his testimony. Then he has this reaction where he says, 'who would have thought the Book of Mormon could do all this?'"
https://witnessesofthebookofmormon.org/three-witnesses/martin-harris/