Hey everyone: can we stop with the personal remarks? There are actually issues here about Christianity, and attitudes about religion in general, that are worth talking about.
I.m.o. the "don't be friends with non-Christians" argument doesn't hold up even if we constrain ourselves to what it says in the New Testament (let's neglect the special case of self-described Satanists for now). Most of the quotes Waymarker brought up, which say to dust off your feet and leave if people reject the Gospel, etc., are specifically in the context of the early church's missionary work. The gospels and Paul are talking about what evangelists should do, when they are going around evangelizing: if people reject your message, move on to the next house, or the next town. (This is almost common sense.) They are not saying that average Christians cannot befriend non-Christians in their day-to-day lives. We know from historical context that when Christianity was just beginning to spread, most Christians had many close non-Christian friends and family, even spouses. The emperor Constantine's wife was Chrsitian long before he converted, for example. And then of course we have all the examples of Jesus praising non-Christians, such as the good Samaritan, and breaking bread with "tax collectors", etc.
Other quotes don't necessarily mean what Waymarker interpreted them to mean. For example, John 15:14 Ye are my friends, if ye do whatsoever I command you .... logically, this does NOT imply that the converse is true: Ye are NOT my friends, if ye do NOT do whatsoever I command you. On the other hand, this converse may have still been intended, since the Gospel writers weren't necessarily well-versed in logic and made logical errors.
Then there is a separate issue: IF there are some things in the Bible which suggest Christians should not be friends with non-Christians, is this in fact an ethical/reasonable teaching?
Now to get back to the issue of "Should a Christian be friends with a Satanist"....First let's just establish that it is perfectly reasonable to choose not to be "friends" with some people. I probably would not accept a friend request from someone who is proudly a member of the Ku Klux Klan, for example. This doesn't mean I wish them harm, I just don't consider that person my friend. The enemy of my friends (blacks, Jews, gays, etc.) is not my friend.
You could argue about the subtleties of whether or not it is better to be friends with everyone, and thereby try to change their minds. That's a good argument. But I think we all have to agree it is at least reasonable, and understandable, to restrict friendship in some cases. No one would call me intolerant, or an egomaniacal jerk, for choosing not to accept a "friend" request from a neo-Nazi; even though it may be the case that this particular "neo-Nazi" is just a harmless, confused and ignorant white guy, who's really a nice person at heart, and by becoming his friend maybe I can help him "see the light". But if this is one of those people who aren't going to "see the light" and are pushing for a racist agenda in the real-world, it's no longer a debate between friends; it's a (peaceful) struggle between opponents, and he's not my friend.
The question, ultimately, is: are we making that choice of "friends" for good reasons? My crieria are generally that my "friends" are honest, tolerant people who support democracy and oppose bullying (preferably a sense of humor). Other than that, I don't care what gods you worship or how you pray.
Within the context of Christianity, Satanism is in principle far worse than racism, or Nazi-ism, or anything else you can think of. It seems to me that within Christianity, therefore, one would understandably be disinclined to be "friends" with a Satanist. The only argument in favor of the "friendship" would be that most "Satanists" aren't really worshipping Satan the way they are depicted worshipping Satan in legends and witch myths, drinking the blood of babies and destroying crops and whatnot. So you could argue that a Christian should be friends with a confused, relatively harmless Satanist, but not with a dedicated one (if you get what I'm saying).
Now of course the question becomes, above and beyond Christianity, from a purely ethical perspective, is this a reasonable or fair way of choosing friendship?