"The verse" is at the end of the one hundred verses Mr. Price forces the girls to write out from the book of Numbers as a punishment when they cause trouble. The hundred verses end on Numbers 32:32, the verse Mr. Price considers most relevant:
We will pass over armed before the Lord into the land of Canaan, that the possession of our inheritance on this side of the Jordan may be ours.
In other words, Mr. Price wants the girls to understand that they must be "armed" with righteousness if their father's missionary work in Africa is to be successful.
In this case, Methusaleh, the family parrot, says the word "damn." Mr. Price is worried that whoever taught the parrot the word is threatening the entire family with God's wrath, asking "why you would have a poor dumb creature condemn us all to eternal suffering." When none of the girls will confess, Mr. Price has the three oldest girls do the writing as a punishment.
In fact, as Adah explains, the parrot learned the word from their mother, who said "damn" over and over again when her Betty Crocker Angel Dream cake mix, brought from the United States, failed in the African climate. The girls' silence saves their mother from their father's anger.
The episode is part of the tragicomic first section of the novel, in which Mr. Price takes upon himself to bring his family to the Congo to try to convert the local Africans to Christianity, all while showing a complete lack of sensitivity to the culture he is entering. The cake mix is emblematic of the adults' failure to understand the culture and environment they are encountering.