I think the Angel Core / Eva Core system was inspired by the Rosicrucians' Rosy Cross
At the beginning of each episode there is a picture of Robert Fludd's Tree-Of-Life. Robert was a member of the Rosicrucian order, which was a European secret society obsessed with Hermetic Qabbalah. The rosy cross was their symbol. It features rose in the middle of the cross, which I think looks like a core.
Furthermore, the descendent organization of the Rosicrucian order, was the Order of the Golden Dawn. Their rosy cross symbol was different. It featured another smaller cross inside the rose. This seems to be a hint at the entry plug system, where there has to be a soul inside the core to allow the Eva to be controlled by the pilot.
