This part of the novel comes in Chapter 14 when David, Petra and Rosalind have been captured by the Fringes men and are being taken back to their base. Whilst stopping, David has a highly revealing conversation with one of his captors about the state of the world. The comment that you have highlighted conveys his idea that creation is continually in a process of transformation, change and development. Note what he says:

God doesn't have any last word. If He did He'd be dead. But He isn't dead; and He changes and grows, like everything else that's alive.

According to this man, the Tribulation was sent to remind the "Old People" of the fact that "life is change" and that they weren't the final word. Of course, the fact that the novel is named The Chrysalids points towards some kind of theme of transformation in process, just as the caterpillar goes through a time in the chrysalis whilst transforming into the butterfly. Clearly the novel presents us with the world and the human species in a similar situation.