The power to wield runes was given by this creature, Gaia, which represents the consciousness of this world. As the core shone with golden lights, the gentle voice spoke to him, teaching him the ways of the world.
Initially, Arthur did not want to listen because he realized that for him to return to Earth means taking away what kept Diana safe. However, the knowledge did not need his consent to transmit into his mind.
Time here was elastic, expanding beyond measure as he wished for it. Arthur could feel the seconds tick by, or he could let them become years, diluting his consciousness until it experienced every moment for a long time.
This was what Arthur did to reach a decision. However, the choice never changed, and he was not his father. Arthur wanted to find a different way to return without taking back what was his.