This is the first of several chapters where Alma and Amulek are preaching to the wicked people of Ammonihah. Remember, this is the second time that Alma has preached to them. He is only returning because the angel of the Lord commanded him to return.
The people are scornful because Alma expects them to listen to the witness of just one man. This is why the Lord sends missionaries in pairs, because people are more likely to listen to the testimonies of two people than just one.
Alma spends most of the chapter trying to impress upon these people the importance of repenting. He uses the Lamanites of an example of those who have turned against the Lord and have been cut off from his presence. However, he tells the Ammonihahites that the Lord will be more merciful to the Lamanites at judgment day than to them, because the Lamanites are wicked because of the sins of their fathers, but the people of Ammonihah should know better, having had so much light and knowledge given unto them of the Lord their God. We too have had so much light and knowledge poured down upon us. That is why the Lord will be much harder on us than on people who never have heard the Gospel and who have never lived its precepts, like we have. There is a lot of responsibility in being members of the Church. The Lord doesn't want half-hearted service, he expects us to serve him with all of our heart.
Alma tells the people that the Lord is basically giving them one more chance before they are destroyed off of the earth. The Lord likes to give his people as many warnings as possible before exacting justice. If we listen to the words of the prophets and follow them, we will be safe.