A ZGMF-X series unit created by ZAFT in C.E 71, the Testament is designed for researching the Earth Alliance's Striker Packs and thus can use them. Like the other ZGMF-X units, it also has Neutron Jammer Canceller and is powered by a nuclear reactor. The Testament's name refers to "a covenant between God and humans", the idea being that Coordinators are gods and Naturals are humans. This name epitomizes the madness and hubris of ZAFT under Patrick Zala.
The Testament is never fielded in combat by ZAFT as it is stolen by the Earth Alliance/The Clan from GENESIS Alpha while ZGMF-X11A Regenerate Gundam is away. Following its theft, the suit is used by a special intelligence unit within the EA and given the model number, RGX-00. It is also upgraded with the Variable Phase Shift (VPS) armor and the Quantum Computer Virus Transmission System. Due to the VPS armor, Testament turns from gray to white after activation, and then to red during close combat. The virus transmission system originated from the NMS-X07PO Gel Finieto, it allows the Testament to infect the quantum computers of nearby mobile suits and hijack them.