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 two radically different motivations, in two radically different spiritual identities, and the result is two radically different kinds of lives.
“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.”
The primary difference is that of motivation. In religion, we try to obey the divine standards out of fear. We believe that if we don’t obey we are going to lose God’s blessing in this world and the next. In the gospel, the motivation is one of gratitude for the blessing we have already received because of Christ. While the moralist is forced into obedience, motivated by fear of rejection, a Christian rushes into obedience, motivated by a desire to please and resemble the one who gave his life for us.
Tim Keller, The Reason for God, pp. 179-180