Christmas and Easter (which he is right to link together) are not just about forgiveness or mercy; they are also about justice. God is both a just God and a forgiving God. If God punishes all sin, then he is perfectly just but without mercy. If he forgives all sin, then he is perfectly merciful but at the expense of justice. That is why God could not, as Richard Dawkins argues "just go right ahead and forgive us".
True forgiveness has a cost, both to the one who forgives (who has to accept a wrong and set it aside) and the one who is forgiven (who has to accept that they have done wrong and deserve punishment). This conflict between justice and mercy is resolved through Christ.
God the Father and God the Son agreed willingly out of love for mankind that Christ would come to earth, knowing that He would be cruelly put to death, to satisfy the need for justice so that God could then exercise mercy. Christ pays the debt of men and women to God, just as someone might pay the fine for a convicted criminal today.
As Paul puts it in his letter to the Romans, "God demonstrates His own love for us in this: While we were still sinners [ie enemies of God] Christ died for us."
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