I have read that befriending non-Muslims is permissible, granted that they do not mock Islam or work against the Muslims. I would like to ask whether it is permissible or haram to be close friends, intimate friends, and/or best friends with non-Muslims, with the conditions listed previously in addition to the Muslim still practicing Islam and not letting the friendship affect their faith.

If you are certain that your faith and spirituality won’t be affected in any way, then yes it’s permissible.