The Tribulation was a catastrophic nuclear holocaust which destroyed the Old People and dramatically altered the environment of the world and its inhabitants. The citizens of Labrador are strictly religious and believe that the Tribulation was God's form of punishing the Old People for their transgressions. The zealous citizens of Waknuk, particularly Joseph Strorm, use the Tribulation to support their arguments for laws enforcing standards of Purity, which discriminate and punish any type of mutations or deviations from the norm. Since the Tribulation, a significant amount of the population is mutated, which the religious citizens regard as "Blasphemies" against the true Image of God. These "Blasphemies" are sent to the Fringes, which is a harsh environment, where everyone with a deviation is forced to live. Uncle Axel also explains to David that the Tribulation has caused some of the barren regions of the Badlands to glow at night, which is another clue that implies it was a nuclear holocaust. The fear of experiencing another Tribulation motivates religious zealots like David's father to maintain their intolerant, austere lifestyle.
The Tribulation exerts a powerful psychological effect on those in charge of Labrador. This self-selecting elite of religious elders are so anxious to avoid another gigantic cataclysm that they presume to determine who should live and who should die: who's "normal" and who isn't. Though no one knows precisely what happened during the Tribulation, it's fair to assume it wasn't pretty. In any case, what matters now is how this event is interpreted and what contemporary significance it holds. The elders in charge of Labrador have made it clear that the Tribulation's ongoing significance lies in the act of warning against the dangers of impurity, in whatever shape or form.
The Tribulation thus acts as a kind of creation myth for this dystopian society. It posits the notion that society can only exist in any manageable form if it is purified of its supposedly inferior elements. This explains why those deemed as mutants are expelled from mainstream society, ritually exterminated, or banished to the harsh wilderness of the Fringes. In carrying out such repressive acts, the elders of the community are using the Tribulation as a means of consolidating their own power and prestige.
Tribulation gives the leaders an excuse to carefully control everyone and create "Purity of the Race" (ch 6).
Tribulation was the holocaust or apocalypse-like incident that wiped out a large part of the population and caused them to start over. The idea of Tribulation is based on the expulsion from Eden as a result of Eve’s sin.
Tribulation had been another such punishment, but the greatest of all: it must, when it struck, have been like a combination of all these disasters. (ch 4)
The people are so caught up in Biblical imagery that they refuse to accept anything that is an anomaly, including people with deviations from the norm. They feel that they are preventing another Tribulation by doing so.
It is easy to control people who are afraid they will offend God if they do anything other than what they are told.
Not everyone believes that Tribulation was a sign to bring humans into a strict God’s-image society. When David and the other Telepaths enter the Fringes, they hear a different point of view from the philosophical man who takes them prisoner.
'Some day,' he proclaimed, 'something is going to steady down out of all this. It'll be new, and new kinds of plants mean new creatures. Tribulation was a shake-up to give us a new start.' (ch 14)
Of course, in the end the Telepaths meet the Sealanders, a group of people who are also telepathic. The Sealanders tell them that their people are living in history, and they are the future.