What is the ruling on returning the greeting of "As-salamu alaykum wa rahmatullahi wa barakatuh" to a person from the People of the Book or an atheist who initiates it in the same form, and what is the extent of validity of the opinion of some preachers who permit it?

The majority of scholars hold that a Muslim should respond to a disbeliever's greeting by saying: "Wa 'alaykum" (And upon you), based on the Prophet's (peace be upon him) saying: "When the People of the Book greet you, say: 'Wa 'alaykum.'" Some scholars are of the view that if a Muslim is certain that a disbeliever said: "As-salamu alaykum" (Peace be upon you), then he should respond with: "Wa 'alaykumus-salam" (And upon you be peace), out of justice and kindness, acting upon the noble verse: "And when you are greeted with a greeting, greet in return with one better than it, or (at least) return it equally."