MM is correct that one doesn't usually use Mr. and a first name unless the speaker is a child speaking of or to an adult. For one adult to speak of another adult that way has a servile ring to it that can be offensive to groups of people who have been disadvantaged in society and who were often relegated to servile roles in the past.
An example is that it was the custom in the Deep So
Thanks for the fuller explanation. I just want to comment that my original remark arose because 'Mr John' (using the first name) is a very common phenomenon among ESL students, so that I now automatically assume the error we have been talking about.
I didn't know it was common among ESL students, but knew exactly what you were talking about. Since this is such a sensitive issue, especially in the part of the U.S. that I live in, I wanted to make certain they understood why it is offensive, and in what context it is not offensive (children to adults). You were definitely right to point out that it is problematic in most ins