Is a godparent who has left the faith still the child's godparent?

You have to go to the church and tell them officially, for example that John Smith, the godparent, has left the faith, and that you need a new godparent. The church will allow you to do that; you bring your candidate to them, they test the person a little bit, and then they will accept it. But you have to go to the church and get it done officially.