He has been a member there 20 years. Which, the way evangelical churches are run in this country means he has had a hand in the management of that church and is responsible, on a small level, for Wright's hiring and retention.
Quitting that church does not make Obama any less responsible for his being there, any less of a racist, and does make him look very dishonest.
The way things are going, he would have been better off admitting he was a racist and moving on. That kind of sleaze is getting more popular all the time.