The Ku Klux Klan is a Christian terrorist group because they engaged in acts of terror. They burned down churches and executed people they considered harmful to their vision of Christian and American ideology.
There still exist people who claim to be Christians and forbid interracial marriages. There still exist people who claim to be Christians and claim white supremacy. There still exist people who claim to be Christians and harbor significant anti-Semitic sentiments. When these people act on those beliefs to terrorize other people by threatening politicians who support racial equality, to burn down black churches, or to hang Jews from a tree, they become Christian terrorists.
While the Ku Klux Klan may have harbored anti-Semitic sentiments, they are best known for their anti-Negro sentiments. There was a common teaching in the 19th Century that white men were the descendants of Seth, Negroes were the descendants of Ham, and Asians were the descendants of Japheth. They saw the domination of the Negroes (Ham) and the incursions of the Asians (Japheth) as fulfillment of the prophecy of Noah that Canaan (a son of Ham) would serve Seth and that Japheth would live in the tents of his brothers. This educated their ideology of white supremacy.
This ideology of white supremacy turned into violence and terrorism after the Civil War. The Negroes were now equals to white men and the KKK felt that this was a violation of the Biblical prophecy, so they acted to suppress the freedmen and their allies. Per the Wikipedia article that you referenced, you see that they threatened and assassinated Negro politicians. This is similar to the actions we hear about on the news performed by Al Qaeda, ISIL, the Taliban, and other terrorist groups in the Middle East.