The Tribulation was some sort of world-ending, population-killing, cataclysmic event. We are told it was equivalent to all of the greatest disasters in the world combined.
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.
The Waknuk people think that God sent the Tribulation to punish mankind for some kind of sin against God or creation itself. The Waknuk people do not want the world to go through another Tribulation, so they have made it a societal goal to stay in line with what supposedly worked for people and God before they were punished with the Tribulation.
Only the authorities, ecclesiastical and lay, were in a position to judge whether the next step was a rediscovery, and so, safe to take; or whether it deviated from the true re-ascent, and so was sinful.
The main rule that now stands as the guiding principle for the Waknuk people is the preservation of the human form.
Above all, he must see that the human form is kept true to the divine pattern in order that one day it may be permitted to regain the high place in which, as the image of God, it was set.
This mentality has led the Waknuk people to be completely averse to natural genetic changes in the population. Anything that is considered outside of the norm is "Deviant." A person is a Deviant if they carry deviant genetic coding, and this coding must be purged from the Waknuk's gene pool. These people are killed, banished, or sterilized, and the resulting population is a population that is fearful of any changes. Consequently, the Waknuk society is a society filled with hidden fears about their fellow members.
From the perspective of the Walnuk people, Tribulation in The Chrysalids is a kind of punishment or trial, a stage of development imposed to motivate people to cleanse their world of all deviation. It is distinctly religious in tone, something sent down from "above," rather than a natural or unnatural disaster. It has the tone of Job's trials in the Bible or the destruction of Sodom and Gomorrah. In order to deal with the Tribulation, an entire body of new "scripture" is developed, to ensure that mutations, of plants, animals, and people, are expunged, so that mankind can emerge once again in the image of God. All rules, regulations, and rituals are centered on this cleansing process.
The Sealanders' perspective is quite different. The Tribulation, according to them, was a phenomenon that mankind brought down upon itself, and the response of the Waknuk people to the Tribulation puts them at an evolutionary dead end. The Waknuks are fighting nature itself, which is inherently always changing and mutating, and their battle to stop change cannot possibly be won. The Sealanders, on the other hand, are celebrating their own evolution to a people who can communicate between minds. Their goal seems to be to work to use this new means to take over the world themselves. Taking in Petra, who is the most highly evolved in this way, is to use her skills to teach others less highly evolved to increase their own power.
While the Waknuks have used the Tribulation to gain power and control over their society in what is portrayed as an evil way, the Sealanders really do not seem all that different. It is not so clear that the Sealanders' "religion" is going to be an improvement upon the old religion.