Revisionist historians argue that the United States was not a Christian country. Here is an official government document from 1779 that is in government archives. The document references the Christian religion and the spreading of the gospel as an important mission for the United States.
Whereas the Honourable the General Congress, impressed with a grateful sense of the goodness of Almighty God, in blessing the greater part of this extensive continent with plentiful harvests, crowning our arms with repeated successes, conducting us hitherto safely through the perils with which we have been encompassed and manifesting in multiplied instances his divine care of these infant states, hath thought proper by their act of the 20th day of October last, to recommend to the several states that Thursday the 9th of December next be appointed a day of publick and solemn thanksgiving and prayer, which act is in these words, to wit…
“Proclamation Appointing a Day of Thanksgiving and Prayer, 11 November 1779,” Founders Online, National Archives, https://founders.archives.gov/documents/Jefferson/01-03-02-0187. [Original source: The Papers of Thomas Jefferson, vol. 3, 18 June 1779 – 30 September 1780, ed. Julian P. Boyd. Princeton: Princeton University Press, 1951, pp. 177–179.]