Complex sentence: The Masters of the Ancient Wisdom are enlightened beings identified by the Theosophists Madame Blavatsky, Henry S. Olcott, Alfred P. Sinnett, and others, who claimed to have met some of them during their lifetime in different parts of the world.
Split into simpler sentences: The Masters of the Ancient Wisdom are reputed to be enlightened beings originally identified by the Theosophists Helena Blavatsky, Henry S. Olcott, Alfred P. Sinnett, and others. These Theosophists claimed to have met some of the so-called "Masters" during their lifetimes in different parts of the world.