UPDATE: Professor Juan R.I. Cole, an Orientalist scholar at the University of Michigan fluent in Arabic, had written,

“People often ... ask me about this verse:

“[5:51] ‘O you who believe, do not take Jews and Christians as friends; these are friends of one another. Those among you who ally themselves with these belong with them.’

“This is actually not a good translation of the original, which has a very specific context. In the Arabia of Muhammad’s time, it was possible for an individual to become an honorary member or ‘client’ of a powerful tribe. But of course, if you did that you would be subordinating yourself politically to that tribe. The word used in Arabic here does not mean ‘friend.’ It means ‘political patron’ (wali). What the Quran is trying to do is to discourage stray Muslims from subordinating themselves to Christian or Jewish tribes that might in turn ally with pagan Mecca, or in any case might have interests at odds with those of the general Muslim community.

“So the verse actually says:

“[5:51] ‘O you who believe, do not take Jews and Christians as tribal patrons; these are tribal patrons of one another. Those among you who become clients of these belong with them.’ ...”

Thanks for clearing that up.