11 Suffer not yourself to be led away by any vain or foolish thing; suffer not the devil to lead away your heart again after those wicked harlots. Behold, O my son, how great iniquity ye brought upon the Zoramites; for when they saw your conduct they would not believe in my words.
Corianton was a missionary who allowed himself to be lured away from his missionary labor by a wanton woman. Alma's words must have stung: "When they saw your conduct they would not believe." Our deeds speak louder than our words. Everyone observes how we act and live our lives. If they see a life of hypocrisy, they may turn from what we believe.
The epistle of James tells us that "A double minded man is unstable in all his ways." You can't profess belief in one thing and then live something else and be stable and at peace. And when you live in ways contrary to the truth you know, you display the kind of hypocrisy that drives people away from the truth. It's hard to separate the message from the messenger.
Corianton is a happy example. He apparently listened to his father, repented, and is mentioned later in the Book of Mormon as a servant of the Lord. (See Alma 49:30)