Jehovah commanded Alma to take Amulek when he went to preach. Of course, he did. We learn why in verse two. The very first question asked was “Who art thou? Suppose ye that we shall believe the testimony of one man?”
The answer will be that they will not have to believe one man. There will be two witnesses. God had already prepared the perfect answer for what God knew would be a principal argument that the people would put forth.
Verse 3 interjects the commentary that the issue of the earth passing away was perhaps a real one. They did not understand the prophecies that the earth would pass away. It is important for modern readers to understand that, although the story of Ammonihah is given to represent a people in rebellion against God, they may have had some legitimate questions. Likewise, there are those in the modern world who will question God’s representatives. They may question some of the statements, and the reason will still be “for they knew not.” Legitimate questions are not necessarily attacks on the faith, but opportunities to explain divine understanding.