An atheist believes that an entire category, or type, of thing(s) does not exist (in this case God/deity) . A theist (someone who DOES believe in a god in one form or another) is someone who DOES believe that category of thing(s) does in fact exist. This is a huge separation. It would be like someone saying “We are both A-Universists (Someone who does not believe in the existence of the universe). I just happen to believe in one less universe than you.” It’s an entire category that one person accepts and one denies. The attempt is to create an illusion of closeness in position, when the fact of the matter is that the theist and atheists are on completely opposite sides of this issue.
The next time an atheist tries to use this line on you, flip the statement on it’s head and say something like “We are both theists, I just believe in one more God”. By doing this, you will show him how silly this statement is to having a real conversation about God.
Don’t let the atheist use the “one less god” phrase against you. Put them in their place and make them recognize that you are worlds (realities?) apart and that saying things like this does not further their case for a creator-less creation. Theists do have a responsibility to defend their position (1 Peter 3:15), but the atheists are just as obligated to put forth arguments for their positions, which, in my view, ultimately fail.