Al-Ma'idah: 14
"And from those who said..."
"We took their covenant"
We took from the Christians a covenant like the one mentioned before them regarding the people of Moses—that is, a covenant similar to theirs—to believe in God and the messengers and to perform good deeds. We took from the Christians their own covenant to do so.
If you ask: Why was it not said "from the Christians"?
I reply: Because they only named themselves that as a claim to be helpers of God (Ansar Allah). They are the ones who said to Jesus, "We are the helpers of God," then they differed afterward into Nestorians, Jacobites, and Melkites.
"So We stirred up..."
We attached and bound them. It is derived from ghariya (to be obsessed/attached) with a thing, meaning to stick to it and be attached to it, and one person inciting another to it. From this comes al-ghira' (glue) with which things are stuck together.
"...among them..."
Between the differing sects of the Christians. It is also said: between them and the Jews, and the like.
"And thus We make some of the wrongdoers allies of others" (Al-An'am: 129).
"Or He confuses you into sects and makes you taste the violence of one another" (Al-An'am: 65).