God gives his child upon his slate a sum-
To find eternity in hours and years;
With both sides covered, back the child doth come,
His dim eyes swollen with shed and unshed tears;
God smiles, wipes clean the upper side and nether,
And says, 'Now, dear, we'll do the sum together!'

George MacDonald

http://www.poemhunter.com/poem/forgiveness-103/