Another "proof" that the C.O. is likely to bring up is the house to house preaching work of JWs. 'Who else is doing that?' he may ask.
You simply tell him that there is not a single account in all the bible portraying Jesus or the apostles systematically going from house to house preaching to people at the door. All the narratives describe public preaching - the open outdoors, the marketplace, the synagogue, the temple, etc.
Interestingly, Jesus' own words at John 18:20 refutes the Watchtower suggestion that he ever went house to house preaching to people at their door.:
"Jesus answered him: “I have spoken to the world publicly. I always taught in a synagogue and in the temple, where all the Jews come together; and I spoke nothing in secret. " - John 18:20
Jesus' direction to his apostles about greeting the household had nothing to do with preaching from house to house, Jesus was schooling them on finding a suitable place of lodging in the village or city that they went in to preach publicly. This is why he tells them not to transfer from house to house - he was telling them not to abandon the hospitality of a modest home owner to go to a more comfortable house.
The showing of hospitality was a privilege in bible times and this is why Jesus told his followers that in whichever village or city they entered, search out who is deserving [of the privilege of being their hosts] and stay there (i.e. lodge there as a guest) until they leave. Jesus did the same thing when he traveled from city to city. He would always stay at the home of someone. You can point to the account of when he went to Jericho and he found Zachaeus to be deserving of the privilege of hosting him. Others in the crowed took offence at Zachaeus - a tax collector - being given this privilege. (Luke 19:1-10)
The passing reference in Acts where Paul speaks about teaching and preaching from "house to house" is likely referring to him visiting one congregation after another. He traveled from congregation to congregation all of which convened in the private houses of believers. Thus his statement of teaching from house to house is essentially another way of saying he taught from congregation to congregation.
But the Greek text actually just says "kata oikon" ("according to house") and may simply mean "in private homes". The exact same Greek expression is used at Acts 2:46 where the NWT translates it as "in private homes". So the earlier Acts reference to the apostles preaching in the temple and "from house to house" (NWT) simply means they preached in the temple and in the homes of believers - at early congregation meetings where they met for prayer and the taking of meals together. (Acts 2:46)
All christian denominations are involved in preaching but not all use the method of house to house and the bible does not mandate the specific method of house to house. So it is dishonest of Watchtower and JWs to suggest that only they are involved in the preaching work. Additionally, the bible shows that, contrary to what JWs claim, not all christians are called to the work of being a formal public evangelizer:
"And he gave some as apostles, some as prophets, some as evangelizers, some as shepherds and teachers," - Ephesians 4:11