Can a Muslim boy marry a Christian girl if allowed by religion?

Yes, A Muslim can marry a girl/woman who is Christian or Jewish and they

can keep their original thoughts.

The holly Islam Book "Koraan" maintains full respect to Christianity and full respect to Jesus (peace upon him) and calls for mutual respect between both religions. A full chapter is mentioned in Koraan about the Virgin Mariam (or Marya).

Again, yes it is possible for a Christian girl to get married with a Muslim man while both maintaining and practicing (at home and/or Church/Mosque) their own religions.