If you don’t want to read the entire book The Reason for God, pick the chapter “Religion and the Gospel.” An excerpt:
There is, then, a great gulf between the understanding that God accepts us because of our efforts and the understanding that God accepts us because of what Jesus has done. Religion operates on the principle “I obey — therefore I am accepted by God.” But the operating principle of the gospel is “I am accepted by God through what Christ has done — therefore I obey.” Two people living their lives on the basis of these two different principles may sit next to each other in the church pew. They both pray, give money generously, and are loyal and faithful to their family and church, trying to live decent lives. However, they do so out of radically different motivations, in two radically different spiritual identities, and the result is two radically different kinds of lives.
In which camp are you?