Verses 1 and 2 represent a typical literary form that Mormon uses. He presents contrasts. In verse 1, we have many believing Samuel, and then going to find Nephi for more instructions and to be baptized.
Verse 2 has two reversals. The first is that there are those who do not believe, and the second is that some of those who witnessed what they deemed a miracle were converted.
The nature of the miracle requires a little explanation, because our visual perceptions of Samuel preaching on the wall comes from the Arnold Friberg painting that was included with the Book of Mormon for so many years. It has Samuel on a very high wall. Those below shoot arrows and cast stones, but it doesn’t seem all that surprising that they miss as he is so far away.
Mark Wright has noted that our perception is the problem. The more likely height of the walls was much lower, perhaps only somewhat above the height of the tallest person. That meant that Samuel was easily visible, and could be easily heard. It should also have meant that he would have been an easy target for stones and arrows. Both slings and bows were military weapons, and in the absence of a standing military, there would have been people round about who were capable with both weapons. Standing only perhaps six feet above the crowd, it was truly a miracle that he could not be hit with the stones or arrows. That was the miracle that was witnessed, and the reason that it was understood as a miracle.