Our sin must be extremely horrible.
Nothing reveals the gravity of sin like the cross. For ultimately what sent Christ there was neither the greed of Judas, nor the envy of the priests, nor the vacillating cowardice of Pilate, but our own greed, envy, cowardice and other sins, and Christ's resolve in love and mercy to bear their judgment and so put them away. It is impossible for us to face Christ's cross with integrity and not to feel ashamed of ourselves. Apathy, selfishness and complacency blossom everywhere in the world except at the cross. There these noxious weeds shrivel and die. They are seen for the tatty, poisonous things they are. For if there was no way by which the righteous God could righteously forgive our unrighteousness, except that he should bear it himself in Christ, it must be serious indeed. It is only when we see this that, stripped of our self-righteousness and self-satisfaction, we are ready to put our trust in Jesus Christ as the Saviour we urgently need.
--From "The Cross of Christ" (Leicester and Downers Grove: IVP, 1986), p. 83. Thanks John Stott.