Why 'the importance of being IN earnest'?

Charles Simeon is one of my great heroes. He is best remembered as the long-suffering and long-serving pastor of Holy Trinity Church in Cambridge, England.

In 1812, Simeon travelled down from Cambridge to London to collect a portrait from India House. As he unpacked it and saw the haggard and worn facial features of the subject, he turned away and cried aloud with enough anguish to prompt passers-by to assume that he must have been the subject's father. But he wasn't. The portrait was of his recent curate, Henry Martyn, who turned his back on a glittering legal career to enter full-time Christian ministry. Martyn left England in 1805 and used his towering intellect to serve the Lord, only to die in 1812, aged 31 yrs old.

Throughout his time as Simen's curate and as a missionary, Martyn received regular encouragements from Simeon to live for Christ and for His people. And yet, it was Martyn who was to have the last word. The portrait that Simeon collected hung in the old man's dining room, over the fireplace. When Simeon's friends visited, he would often point at the picture and say: "There, see that blessed man! What an expression of countenance! No one looks at me as he does; he never takes his eyes off me, and seems always to be saying, 'Be serious - be in earnest - don't trifle - don't trifle.'" And then smiling at the picture, and gently bowing, Simeon woul say, "And I won't trifle - I won't trifle."