1 Corinthians 6:7-8
To have lawsuits at all with one another is defeat for you. Why not rather suffer wrong? Why not rather be defrauded? But you yourselves wrong and defraud, and that even your own brethren.
The Church at Corinth was a living laboratory of plain-as-the-nose-on-your-face reasons why a sacrament of reconciliation is necessary after baptism. For the simple truth is that baptism is grace, not magic. It takes away our sin, but it does not take away our capacity for sin. The Corinthians (and pretty much all the rest of us) made a big career of sinning after baptism. And in so doing, they showed that being born again, like being born, is the beginning, not the end, of the story. As the story continues, the hero gets hungry, eats, and then gets hungry again. So God gives him the daily bread of Eucharist. He gets sick and needs to get well, so God gives the sacrament of anointing. He needs to grow up, so God gives him the sacrament of confirmation. He sins and needs forgiveness all over again, so God gives him the sacrament of reconciliation. If you've sinned and need to be reconciled to God, don't waste time kicking yourself, get to confession. God will be happy to have you back!