How did the Ku Klux Klan define Americanism?

One would hope that the Ku Klux Klan did not define "Americanism" per se, as a whole. But unfortunately it did represent that part of Americanism that has, from the beginning 400 years ago, been defined by a wish to oppress non-white people and to keep them in a position subordinate to the whites in perpetuity.
The positive thing about American society is that in spite of this endemic racism, genuine progress has been made in stages over the centuries. Many scholars believe that it would have been better for African Americans if the Revolution of 1775–1783 had not succeeded. This is a complex issue, given that the British at times made efforts to free the enslaved people if they would fight for the Crown. However, in the aftermath of the successful war for independence, all of the northern states passed gradual abolition laws, and the Northwest Ordinance provided for the exclusion of slavery from the Northwest Territory. So progress of a kind was made, decades before the abolitionist movement got into full swing and the Civil War did liberate the enslaved people as a whole.
Though the defeated South, the former Confederacy, accepted the abolition of slavery, it is tragic that they refused to allow the integration of African Americans into society and that they passed the oppressive Jim Crow laws creating the segregated world that lasted for nearly another century. The Ku Klux Klan was in the vanguard of "enforcing" this oppressive situation, carrying out raids to persecute and terrify the freed people. The blazing cross was a perversion of Christianity but also symbolic of Americans' hypocritical use of religion through the ages to justify not only slavery but other forms of exploitation. The actions of this terrorist group were, in fact, the essence of that aspect of America or Americanism which had deliberately submerged non-whites in the social hierarchy, first through slavery and then through laws disallowing them from living together and in equal status with the whites.
As bad as America has been in this regard, and still is in terms of institutionalized racism, ongoing police brutality, and barely disguised appeals to racist attitudes in the public from the highest levels of government, things have still changed for the better. These positive changes are a form of "Americanism" as well. So the Ku Klux Klan defined only the worst of what America was or still is.