Ask a JW (or any similar) if they ever wonder how other people can be devoutly loyal to their church but still be wrong. (of course they believe them to be wrong)
They will likely say they were "lied to, deceived" because they haven't really deeply considered what makes a person devoutly loyal to a religion. Instead of getting mired in a debate about doctrine with them ask :
Might culture and family have had an influence?
Might it be a sense of community, commonality even affections members of the church have for each other?
Might it be the religion gives them a sense of certainty in a changing world?
Might it offer comfort in times of grief?
Then ask if those are also true of their religion, does it offer certainty and comfort? does it have a warm familiarity from years of memories with family and friends? Do you feel a part of the community of worshipers?
So might what you believe to be true of others be true for you?