As the previous person suggested, the conversation is between Nicholas, the boarder, and John, the homeowner and old husband to the young and beautiful Allsion. The plot is correct in which Nicholas and Allison want to engage in adult activities obviously without John's knowledge.
The quote itself stems from Nicholas' role in instigating actions that will lead to he and Allison engaging in the adult activities.
Remember that the time period and situation in England allows for pretty much anything to be done in the name of Christ or God. Therefore, because Nicholas uses "Christ" it will assure John that the issue at hand is "for real!"
The explanation of the quote is as follows...
"Now John...honour here" = by addressing him by name, the speaker is being respectful and showing the listener that he thinks highly of him; then of course he (Nicholas) uses compliments to gain the attention of John and even goes so far as to ask him to put his honor on the line - which is a basis for the Code of Chivalry.
"Not to repeat a syllable ...Shall fall upon you."= Nicholas then tells John not to repeat anything about the issue at hand (which is going to be Nicholas' prediction of a great flood in the meterological forecast) but not because Nicholas says, but because Jesus Christ says (remember, this makes everything more believable due to the "intensity of Christianity and its press on society").
"You'll be driven mad" = To go against what Christ says will cause John to be persecuted, tortured, or to "pay for his sin" which Nicholas tells him is going to be insanity.
Obviously the role of this quote is to convince John to abide by the plan that Nicholas (who is a credible source because of his "scholarly background" and who had been "faking" intense mental breakdown in his room prior to this conversation).
John then replies,
"Christ and...lad" = basically John swears to God and Christ that he will not say anything about the issue at hand because that is what God wants.
"I'm not given to gab." = John states that he doesn't gossip or really talk to anyone about random issues.
"Say what...harrowed hell" = John reiterates to Nicholas that he can tell him anything he wants, because John will not repeat it to anyone, man, woman or even child of God/Jesus who basically died for every Christian on the cross (harrowed hell - took the worst punishment for others sins).