John Stott:

The concept of substitution may be said to lie at the heart of both sin and salvation. For the essence of sin is man substituting himself for God, while the essence of salvation is God substituting himself for man. Man asserts himself against God and puts himself where only God deserves to be; God sacrifices himself for man and puts himself where only man deserves to be. Man claims prerogatives that belong to God alone; God accepts penalties that belong to man alone. (Taken from The Cross of Christ).