It seems that it wasn't just about them being Christian, but also about them being American as well:
They said when the officer realised they were American ... he launched a tirade against President Bush and the wars in Iraq and Afghanistan.
"He said we were in a Muslim area and were not allowed to spread our Christian message. He said we were committing a hate crime by telling the youths to leave Islam and said that he was going to take us to the police station."