Corianton had gone astray. That does not mean that he was past salvation. Alma clearly believed that he could still repent, and we will learn that he will, indeed, repent and become a faithful servant. Nevertheless, there is a conceptual problem with the true principle of repentance. While repentance can absolutely lead to the same blessings as those who had no need of repentance, there is a danger that one might justify oneself in continuing to commit sin with the expectation that we might simply repent in the future.
That very caution is manifested impactfully in verse 10: we cannot be restored from sin to happiness. The kind of happiness described is the kind of joy that was the result of eating the fruit of the Tree of Life in Lehi’s dream. It is that kind of joy, or happiness, that is the goal. Sin simply cannot get us there. Some sins might temporarily be more enjoyable than righteousness, but their resulting happiness is temporary and artificial.
Alma cautions Corianton to take care to sin no more, else his process of repentance will be more difficult. If we put off our repentance too long, then our characters will be formed in such a way that divine happiness has not become our goal, and repentance will be that much more difficult; not impossible, but more difficult.