A. Publicly Released Information
B. Generally Recognized Information
NERV's personnel are told that, if an Angel were to come into contact with Adam, an explosion of similar magnitude to Second Impact will occur. In actuality, the Human Instrumentality Project would be set in motion.
C. Confidential Information
It is thought that Third Impact will occur if an Angel comes in contact with either Lilith or Adam.
D. In-Depth Information
The alleged reality of Third Impact is that humans will lose their personal boundaries, dissolving as a result. Life will arrive at its terminus (although it is unclear what such an end entails), and A.T. Fields will be lost, rendering the preservation of human form impossible.