Christian Church’s Understanding of Forgiveness

How has the contemporary Christian church’s understanding of forgiveness developed in relation to Christ’s atonement?

Why have Christians grown to the point that they can no longer forgive? Whenever did Christians become so puffed up that they can’t extend mercy to others? What are the fundamentals of Christ’s atonement? What does Jesus require of his followers in terms of mercy and forgiveness? How people’s flawed understanding of mercy has damaged the church of today?