Tertullian said that there are two thieves alongside the Gospel of Christ’s Grace. The thief of legalism threatens to steal the grace of the Gospel and the thief of relativism threatens to steal the truth of the Gospel.
Legalism devalues the grace poured out on the Cross and the fact that all was accomplished on the Cross. Relativism waters down the gruesomeness of the justice poured out on Jesus for our sin. They say that God accepts us just the way we are.
The Gospel says, “because I am accepted I will obey.” Better, “because I have been given a new heart, how can I not want to obey.” There is an inner-reality that is worked out in the life of the believer that works itself out through love and good deeds. The true Christian knows that whatever he does, he will not merit the acceptance he has in Christ. However, he also knows that “children obey your parents” is not just a helpful admonition, but a command. We, as children, are born unto a living hope as it manifests itself through obedience. It is not the root, but the fruit of the seed sown in the soil. Christ’s work is the root and our working is the fruit.
There is a great article by Tim Keller that deals with this topic as it relates to the Gospel in the postmodern culture. Here is a quote from it that was especially impacting:
The gospel produces a unique blend of humility and boldness/joy in the convert. If you preach just a demanding God, the listener will have “low self-esteem”; if you preach just an all-loving God, the listener will have higher self-esteem. But the gospel produces something beyond both of those. The gospel says: I am so lost Jesus had to die to save me. But I am so loved that Jesus was glad to die to save me. That changes the very basis of my identity–it transforms me from the root. Legalistic churches reform people’s behavior through social coercion, but the people stay radically insecure and hyper-critical. They don’t achieve the new inner peace that the grace of God brings. The more relativistic churches give members some self-esteem and the veneer of peace but in the end that is superficial too. The result, Archibald Alexander said, is like trying to put a signet ring on the wax to seal a letter, but without any heat! Either the ring will affect the surface of the wax only or break it into pieces. You need heat to permanently change the wax into the likeness of the ring. So without the Holy Spirit working through the gospel, radically humbling and radically exalting us and changing them from the inside out, the religion either of the hard or soft variety will not avail.