The Kybalion appeared in Chicago in 1908, published anonymously by Three Initiates, and presented itself as the distilled teaching of Hermes Trismegistus preserved in an oral tradition. Its seven principles, mentalism, correspondence, vibration, polarity, rhythm, cause and effect, and gender, have been repeated in occult writing ever since, and they are what a great many modern readers mean when they say Hermeticism.
It was published by Atkinson's own Yogi Publication Society. Its vocabulary, its cadence and its arguments match his signed New Thought books closely, and Philip Deslippe's edition of 2011 sets out the bibliographical case for his authorship in detail. That attribution is the standard scholarly position and it is what the atlas records.
Standard is not unanimous. Some accounts add Paul Foster Case and Michael Whitty as the other two initiates; others read the three as a rhetorical device rather than as three people. The evidence for the additional names is markedly weaker than the evidence for Atkinson, so the atlas marks the ascription disputed rather than closed.
One further point is worth keeping in view. The Kybalion is a New Thought book of 1908, not an ancient Hermetic text, and its seven principles are not found in the Corpus Hermeticum. Saying so takes nothing away from its influence, which has been enormous, but it puts it in the right century and stops a modern synthesis being read as an ancient inheritance.