The simple answer is that the "you have to be good" Christians are not the same people as the "Jesus forgives no matter what" Christians. Beliefs and doctrines vary wildly throughout Christianity, and different Christians often believe contradictory things. This isn't helped by the fact that the Bible itself, being a collection of many books by many authors, contains contradictory viewpoints. This allows believers to focus on the elements they like and ignore the ones they don't.