Keller puts it this way: "The behavior of Christians...has undermined the plausibility of Christianity for so many people." Every day, ordinary people who are also Christians are failing to meet the standards God sets for us. But what people on the outside don't see, as Keller explains, is that being a Christian doesn't mean you are the best of the best. Instead, it means that you realize just how messed up you are and that you need God's grace because "our moral attempts are too feeble and falsely motivated to ever merit salvation."
Now, I'm not an expert on a lot of other religions, but it seems to me (although I may be wrong) that a lot of other religions emphasize the necessity of winning approval from God or the gods, from praying a certain way or a certain number of times or doing good works or not doing certain things. And in some ways, Christianity does, too, through the commandments and teachings. And don't get me wrong - these are so important. But they aren't how we are saved.
What does this mean? It means that Christians are going to mess up. Some people won't do "enough." Some people will do "too much," and become the people more focused on doing the Christian-y looking things than living out the love and acceptance that Christ teaches; Keller calls these "fanatics." All of these things, from the outside, make Christianity seem hollow. Because what is the point if the Christians themselves aren't doing what they tell others to do?
The point is: that isn't the point.
Although the church and the people that comprise it are ultimately epic failures, that right there is the point of Christianity. "The church is a hospital for sinners, not a museum for saints." And that's why everyone is welcome. We all need a little Jesus-medicine.