Yesterday's Gospel was from Luke 15 - "there will be more joy in heaven over one sinner who repents than over ninety-nine righteous people." While you've no doubt heard countless homilies on this subject, and the prodigal son parable, that all focus on how great repentance is, there's a danger.
People come to think of sin as being not very bad because hey, look how much joy it brings when I repent. You can look at these verses and think that it is "OK" or actually even preferable to sin. After all, the ninety-nine righteous people didn't bring God as much joy as me when I sin and then repent. But consider the converse. If it is not true that God delights in sin, and it is true that the repentant sinner brings more joy than ninety-nine righteous people, then the repentant sinner is a net loss, even considering the joy when he repents. Or in other words, one sin brings more sadness than the joy that repentance brings, which we are told is immense.
So next time I think about a sin being "no big deal" I'm going to reflect on how much sadness one sin can bring about. A sobering thought.