really ... not official? Recent (50's) I know, but not official? I'd like to see your evidence.
Here is mine:
https://en.wikipedia.org/wiki/Pledge_of_Allegiance#Addition_of_"under_God"On October 6, 1954, the National Executive Committee of the American Legion adopted a resolution, first approved by the Illinois American Legion Convention in August 1954, which formally recognized the Knights of Columbus for having initiated and brought forward the amendment to the Pledge of Allegiance.
but as you know, I'm not American so I don't know for sure.
https://www.pbs.org/wgbh/americanexperience/features/godinamerica-white-house/
So perhaps you can see why as an outsider that it could be misunderstood that America was a Christian Nation.