The Secret Chiefs · a Third Order nobody could reach

In the Golden Dawn the top three grades were not attainable in the flesh. They belonged to the Secret Chiefs — superhuman or discarnate adepts from whom Mathers claimed to receive the Second Order's teaching, and whom he described meeting in terms that varied over the years.

It is structurally elegant and structurally convenient: an authority that cannot be checked, held by beings who cannot be produced, transmitted through the one man who can reach them. When Mathers' authority was challenged in 1900 this was exactly the ground the fight was fought on.

Crowley's A∴A∴ later made those three grades attainable — Magister Templi, Magus, Ipsissimus become things a living person can claim. That is a different claim entirely, and a much larger one, and the door says so rather than blurring the two systems together.