I know you were just offering it as an example, but you should be relieved to know that the scenario is an extremely unlikely one.
Not only does Judaism not promote proselytizing, it mildly discourages, though does not prohibit, converts. My understanding is that Christians proselytize out of a belief that only those who have accepted Christ can be saved.
Jews believe that Christians who lead a good life can also have eternal life. Therefore there's no need to convert people to Judaism.