If you could name one non-Christian country, group etc. that abolished slavery I would be most obliged.
USA
It has a secular Constitution.
"Congress shall make no law respecting an establishment of religion, or prohibiting the free exercise thereof; or abridging the freedom of speech, or of the press; or the right of the people peaceably to assemble, and to petition the Government for a redress of grievances."
And correlation is not causation. Wording matters, too.
Self-identified
Christians make up 63% of the U.S. population in 2021 according to Pewresearch. But "Christian" is a broad umbrella encompassing many different denominations, sects and cults, some of whom differ so wildly with others that they war with each other over their beliefs (though they don't carry out their wars so much in the US).
Many survey respondents probably think of themselves as christian (adjective), and so answer in the affirmative. I was told as a tot that when it came to food I should have "Catholic taste', meaning new or different foods were to be embraced and appreciated. Having a "Christian" attitude of kindness to others was more important than anything about a church.
By any definition though, The United States of America is not a "Christian Country", its many Trumpsuckers' self identification notwithstanding.