It is likely that a lot of the Scots and Scotch/Irish were rednecks. (Freedom loving Scottish dissidents)
The origins of this term Redneck are Scottish and refer to supporters of the National Covenant and The Solemn League and Covenant, or “Covenanters”, largely Lowland Presbyterians, many of whom would flee Scotland for Ulster (Northern Ireland) during persecutions by the British Crown. The Covenanters of 1638 and 1641 signed the documents that stated that Scotland desired the Presbyterian form of church government and would not accept the Church of England as its official state church.
Many Covenanters signed in their own blood and wore red pieces of cloth around their necks as distinctive insignia; hence the term “Red neck”, (rednecks) which became slang for a Scottish dissenter*. One Scottish immigrant, interviewed by the author, remembered a Presbyterian minister, one Dr. Coulter, in Glasgow in the 1940’s wearing a red clerical collar — is this symbolic of the “rednecks”?
Since many Ulster-Scottish settlers in America (especially the South) were Presbyterian, the term was applied to them, and then, later, their Southern descendants. One of the earliest examples of its use comes from 1830, when an author noted that “red-neck” was a “name bestowed upon the Presbyterians.”
You are correct about “rednecks.” The term began to mutate in the early 1900s when socialist “progressives” who were pushing a form of “social justice” Christianity without God mocked those who believed in the Bible as ignorant, backwards “rednecks.” The battle culminated in the Left’s takeover in the early 1900s of the Presbyterian Church and the purge from Princeton seminary of Christian professors.
The purged professors had been advocating that socialist views had to be rejected and that the Church had to return to the “fundamentals” as set forth in the Bible. Those purged from the Presbyterian church by the Socialists were also disparagingly called “fundamentalists,” so that is where that term came from as one being used to denigrate Christians.
The descendants of those leftists pretty much gained complete control of all the mainline denominations by the 1950s and 1960s, and you can see the results today.