User blog comment:BlackWizardZeref/Fairy Tail Chapter 384: Attack of the Celestials Review/@comment-7418318-20140516225538/@comment-7418318-20140517001256

It's not just about revealing the secret. Aquarius very clearly said that the summoning itself would only work if the bond of trust is strong enough. So even if a less devoted Spirit Mage somehow learned about the method, they wouldn't be able to do it.