What is the name of the primary symbol, which is now outlawed outside of the context of art or science, and was widely used even on the state flag?
Strafgesetzbuch section 86a The German "Strafgesetzbuch" (Criminal Code) in § 86a outlaws "use of symbols of unconstitutional organizations" outside of the contexts of "art or science, research or teaching". This concerns Nazi symbolism in particular and is part of the denazification efforts following the fall of the Third Reich.
Christian Flag The Christian Flag is an ecumenical flag designed in the early 20th century to represent all of Christianity and Christendom. It is used by many Christian traditions, including the Anglican, Baptist, Mennonite, Methodist, Moravian, Lutheran, Presbyterian, Quaker, and Reformed, among others. North America, Africa and Latin America are regions of the world where the flag remains popular. The flag has a white field, with a red Latin cross inside a blue canton. The shade of red on the cross symbolizes the blood that Jesus shed on Calvary. The blue represents the waters of baptism as well as the faithfulness of Jesus. The white represents Jesus' purity. In conventional vexillology, a white flag is linked to surrender, a reference to the Biblical description of Jesus' non-violence and surrender to God. The dimensions of the flag and canton have no official specifications.