The Four Horsemen of the Apocalypse

In this episode, we meet the infamous "four horsemen," the chief causes of the great tribulation we experience in mortality: unrighteous dominion (the white horse), war and contention (the red horse), economic injustice (the black horse), and disease and death (the pale horse). 

The four horsemen are also the autumn and winter signs of the zodiac in the night sky.  John knew this, and he used this symbolism to help people understand why they encounter tribulation in this world.