Gray had never had the chance to see exactly where the marks on the other Great Old Ones and the Sefirah's blessed were located. But as someone who wielded the City of Calamity's power, she had received a blood-red, banner-shaped mark between her brows long ago, in that timeline that led to the apocalypse, where everything ceased to exist.
It was proof that the City of Calamity had answered her and bestowed its power upon her.
In this world, the Red Angel Medici had likewise manifested a similar mark between his brows when praying for the power of the source of war. This meant that the City of Calamity's Sefirah mark, at least, belonged between the eyes rather than in the valley of the chest.
Yet when Angel advanced to an Unaging Demoness, truly met Gray, and received the City of Calamity's Iron-Blooded Knight boon together with her, the mark bestowed by the Sefirah had appeared on her chest instead.
At first, neither Gray nor Angel had paid this any mind. After all, unlike the banner between the brows, a mark on the chest could be concealed beneath clothing. But after learning that the City of Calamity had once been accommodated by that Pillar-level Great Old One, and after Gray recovered all her memories and understood that she had come from beyond the apocalypse, they began considering the reason and purpose behind this "anomaly."
Combined with the overlap between her previous life's faith in the Goddess of Light and the authorities and symbolism held by the Omniscient and Almighty, as well as the inexplicable circumstances in which Angel's physical body had entered the City of Calamity after she died at a devil's hands, the answer was all but obvious.
All of this had likely been arranged by the primordial God