Is it permissible for me to say "wa rahmatullah (and the mercy of Allah)" when returning the greeting of non-Muslims, and is the close relationship a Muslim forms with Christians permissible or forbidden?

When returning greetings to Christians, the majority of scholars hold the view of limiting the reply to "wa 'alaykum" (and upon you). Some scholars permit saying "wa 'alaykum as-salam" (and upon you be peace) without adding "wa rahmatullah" (and God's mercy), while Ibn al-Qayyim holds that it is permissible to return the greeting in kind if it is confirmed that they said "salam 'alaykum" (peace be upon you).

As for befriending disbelievers and taking them as close associates (bitanah), this is a mistake and a forbidden allegiance. One must advise a friend about the obligation of aligning with believers and disavowing disbelievers, supporting this with the Almighty's saying: "O you who have believed, do not take as intimate associates those apart from yourselves." The Prophet, peace and blessings be upon him, also warned against associating with non-believers, saying: "Do not accompany anyone but a believer, and let no one eat your food except a pious person," and "A man is on the religion of his intimate friend, so let one of you look at whom he befriends."