Where does White Nationalism come from? The answer is complex and the sources of ideas for it are many, varied and not all of them are related to each other. But at least some of the rationale comes from religion.
For those in some degree of denial, please be aware that there is some biblical basis for the racism of White Nationalists; the story of Phineas, who murdered an inter-racial couple in cold blood. Some racists refer to themselves until today as "Phineas Priests." https://www.splcenter.org/fighting-hate/extremist-files/ideology/phineas-priesthood
God apparently did not approve of inter-racial marriage. This story is not lost on racist leaning Christians. Here is an excerpt from the Old Testament in the Book of Numbers:
6 Then an Israelite man brought into the camp a Midianite woman right before the eyes of Moses and the whole assembly of Israel while they were weeping at the entrance to the tent of meeting.
7 When Phinehas son of Eleazar, the son of Aaron, the priest, saw this, he left the assembly, took a spear in his hand
8 and followed the Israelite into the tent. He drove the spear into both of them, right through the Israelite man and into the woman’s stomach. Then the plague against the Israelites was stopped;
9 but those who died in the plague numbered 24,000.
10 The Lord said to Moses,
11 “Phinehas son of Eleazar, the son of Aaron, the priest, has turned my anger away from the Israelites. Since he was as zealous for my honor among them as I am, I did not put an end to them in my zeal.
12 Therefore tell him I am making my covenant of peace with him.
13 He and his descendants will have a covenant of a lasting priesthood, because he was zealous for the honor of his God and made atonement for the Israelites.”