Merlin learned of the God's Power test ritual from talking to True God Ainuer.
The God's Power test was the proof of turning the "rules" within one's body into "laws."
Originally, Demigods cultivated Global Rules. As for the direct-lined Demigods, they tended to focus on one rule so extremely that it would have a power that was close to the power of a rule that had the Spirit Deity Seal as a container. To prove the power of the Demigods of the direct line, therefore, one needed to use the Spirit Deity Seal to connect with the universe.
As for the variable-lined Demigods who cultivated one hundred thousand rules, even if they didn't have a Spirit Deity Seal, they could use the hundred thousand rules to create a formation as a connection to take the test of the universe.
However, no matter which path they chose, they had to choose a law to prove their God's Power.