While our nation is still predominantly Christian, that is not something that was built into the country. Most of the countries of Europe had state churches that severely limited the opportunities of those who followed any other religions. The founding fathers were absolute in their desire to avoid that situation. That was why they specified that church and state were to be separate. They did not want to mandate a specific religion... not even Christianity.
It is true that some colonies, like the Pilgrims, came for religious reasons. But they were so limited in their own views that they only wanted colony members who believed as they did. That's why they drove out Roger Williams and his Baptists so that they formed Providence, Rhode Island. Maryland was established as a haven for Catholics while William Penn brought his Quakers to Pennsylvania. True, all those are Christian.
But by the time the colonies were being welded into a nation those writing it into being were more concerned with the nation not being identified with a specific religion than otherwise. (Other than Jews, they probably would not have visualized followers of other religions settling into their new country.)
History and our Constitution are clear about these facts. So, while I am a Christian and so is virtually everyone I know personally, I am not offended when someone says we are not a Christian nation. We were never meant to be.