In the name of Allah , Most Gracious, Most Merciful
http://theislampath.com/smf/Smileys/default/salam.gif
It does not say Christians are pagans . Let me explain the context.
A group of Christians visited Prophet Muhammed (pbuh) at the mosque in Madina. They stayed there for 3 days & nights & had a debate with him over if Jesus (pbuh) is God or not.
Debate went on & on ; then God asked the Prophet to challenge those Christians . Prophet went to the open field with his daughter Fatima , cousin & son in law Ali (ra) , grandsons & asked Christians to say that if Jesus (pbuh) is not God , then curse of God will be upon us & all family members.
another related verse :
O people of the Scripture (Jews and Christians)! Do not exceed the limits in your religion, nor say of Allah aught but the truth.
The Messiah Iesa (Jesus), son of Maryam (Mary), was (no more than) a Messenger of Allah and His Word, (“Be!” - and he was) which He bestowed on Maryam (Mary) and a spirit (Rooh) created by Him; so believe in Allah and His Messengers.
Say not: “Three (trinity)!” Cease! (it is) better for you. For Allah is (the only) One Ilah (God), Glory be to Him (Far Exalted is He) above having a son.
To Him belongs all that is in the heavens and all that is in the earth. And Allah is AllSufficient as a Disposer of affairs.
( سورة النساء , An-Nisa, Chapter #4, Verse #171)