Mosiah 26:6

For it came to pass that they did deceive many with their flattering words, which were in the church, and did cause them to commit many sins; therefore it became expedient that those who committed sin that was in the church, should be admonished by the church.

They tricked many people in the church with nice words. They made them do bad things. So the church needed to warn those who were doing wrong.

These people used smooth words to deceive church members and led them into sin. The church realized they had to confront those who were sinning.