Is it permissible to greet someone whose religion is unknown, whether Muslim or disbeliever, or to return their greeting?

The command to greet "those you know and those you don't know" is specific to Muslims or those who outwardly appear to be Muslim. It is forbidden to initiate greetings with Jews and Christians, and if they greet you, say "wa 'alaykum" (and upon you). In this era, it is difficult to distinguish between a Muslim and a non-Muslim due to imitation in dress and appearance. Therefore, if someone who resembles polytheists greets you, say "wa 'alaykum" and do not initiate the greeting yourself due to doubt about their status. Advise them to abandon imitating disbelievers.