I've actually never before heard that a Christian cannot get into jannah(heaven). There is even an ayah of the quran stating such:

"Indeed, those who believed and those who were Jews or Christians or Sabeans [before Prophet Muhammad] - those [among them] who believed in Allah and the Last Day and did righteousness - will have their reward with their Lord, and no fear will there be concerning them, nor will they grieve." Quran 2:62

Of course the ultimate decision-maker is God, so the exact conditions are up to him. Some would argue Christianity isn't monotheistic, and therefore it conflicts with basic Islamic tenant of monotheism. I don't know and don't see a reason it matters to me-- it is up to Allah SWT.

This does reveal something important, though. Islam is simple, and yet it's also immeasurably complex. There is no central authority that decides what is right and wrong like the pope. It's simply up to us to uphold the truth. But there are plenty of issues where people disagree such as this one. I'd suggest doing some reading as well to solidify your understanding. Try gainpeace.com