Is it permissible to respond to Christians with "wa rahmatullahi wa barakatuh" (and the mercy of Allah and His blessings), knowing His saying, Exalted is He: "And whoever desires a religion other than Islam, it will never be accepted from him, and in the Hereafter he will be among the losers"?

Scholars have differed on responding to disbelievers if they greet with the known form of salutation. Many of them held the view of limiting the response to saying: "Wa 'alaykum" (And upon you). Ibn al-Qayyim, however, held the view that it is permissible to respond with: "Wa 'alaykum as-salam" (And peace be upon you), if the Muslim is certain that he heard the Shari'ah-compliant greeting from them. As for supplicating with mercy, it is not prescribed. And one should refrain from saying: "wa rahmatullahi wa barakatuh" (and the mercy of Allah and His blessings).