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.