The Second Coming by William Butler Yeats.

The best lack all conviction, while the worst are full of passionate intensity.

Turning and turning in the widening gyre   
The falcon cannot hear the falconer;
Things fall apart; the centre cannot hold;
Mere anarchy is loosed upon the world,
The blood-dimmed tide is loosed, and everywhere   
The ceremony of innocence is drowned;
The best lack all conviction, while the worst   
Are full of passionate intensity.

Surely some revelation is at hand;
Surely the Second Coming is at hand.   
The Second Coming! Hardly are those words out   
When a vast image out of Spiritus Mundi
Troubles my sight: somewhere in sands of the desert   
A shape with lion body and the head of a man,   
A gaze blank and pitiless as the sun,   
Is moving its slow thighs, while all about it   
Reel shadows of the indignant desert birds.   
The darkness drops again; but now I know   
That twenty centuries of stony sleep
Were vexed to nightmare by a rocking cradle,   
And what rough beast, its hour come round at last,   
Slouches towards Bethlehem to be born?

Source: The Collected Poems of W.B. Yeats