Hi and welcome swamboozled.
Atypical, I love your question. I'm going to use that in the future. Please don't be offended if I forget to mention that it came from an avid apostate :)
Honesty, that is good too. However, the MIL can counter with the canned WT response:
*** w51 12/1 p. 734 Questions From Readers ***
Not all believers in Jesus followed him along with the twelve apostles. Some who wanted to follow Jesus were told to go back home and bear witness to him there. (Mark 5:18-20) Hence it was not necessary for this man to bodily follow Jesus to be on his side. There were only two sides in this controversy, either for or against Jesus, and since he was not against him he was for Jesus. From Pentecost and the outpouring of the spirit on the faithful it would be necessary for this man to associate himself with the congregation of Christians in order to receive the spirit and be approved of God for not being against Jesus. It is different with the religious systems that now preach in Jesus’ name. It cannot be said that all these are not against him for that reason, for they are against Jehovah’s faithful witnesses who do preach Jesus and his kingdom. So as they are against the least of these his brothers, they are against him and their mere use of Jesus’ name does not gain favorable recognition of them as true followers.
Perhaps someone has a good rebuttal for the WT argument. I notice they make the statement "that from Pentecost and the ourpouring of the spirit on the faithful it would be necessary for this man to associate himself with the congregation of Christians in order to receive the spirit and be approved by God" - I don't actually see any warrant for this statement In the complete QFR article they use Matthew 7: 20-23 "
"Really, then, by their fruits you will recognize those men. Not everyone saying to me, ‘Master, Master,’ will enter into the kingdom of the heavens, but the one doing the will of my Father who is in the heavens will. Many will say to me in that day: ‘Master, Master, did we not prophesy in your name, and expel demons in your name, and perform many powerful works in your name?’ And yet then I will confess to them: I never knew you at all. Get away from me, you workers of lawlessness." - I don't believe that is evidence that can be used to warrant the statement that the WTS makes in the article. I thought Matthew was talking about individuals who had works but had not shown faith and weren't concerned with the sheep just their outward appearance - like the pharisees of Jesus' day. Just mho.