It depends upon the definition. Would we be a Christian nation defined by a legislative fiat? No! That is expressly forbidden by our Constitution in the first amendment to it. Lawmakers shall make no law with respect to religion.
There are 13 countries that do have an official state religion where the church is an integral part of their government. We have no national religion. In fact, one of the reasons the pioneering people who came to America was to escape such a mandated system of beliefs, faith and practices.
Freedom of religion is a basic right of all citizens under our Constitution with the Bill of Rights. Some protestant colonies, early on, assessed taxes upon their citizens to support their churches, a practice that ceased with adoption of the constitution.
We are free to believe what we wish without government interference.
It was the practice of religious faith in Christianity that carried our national forefathers to achieve the basic values and moral courage to write and propose the basic form of government that we have.
Though those of our countrymen who are not Christian still benefit from those basic tenants that give us the core of our national ethos.