Complex sentence: The forerunner of the German Christian ideology was that of Protestant groups of the German Empire, that sought a return to perceived Völkisch, nationalistic and racist ideas within traditional Christianity to turn Christianity in Germany into a reformed "arteigenen Volksreligion".
Split into simpler sentences: The forerunner of the German Christian ideology came from certain Protestant groups of the German Empire. These groups sought a return to perceived Völkisch, nationalistic and racist ideas within traditional Christianity, and looked to turn Christianity in Germany into a reformed "arteigenen Volksreligion".