The Secretum Nobilissimorum et Verissimum (The Most Noble and Truest Secret) is a sixteenth-century alchemical text by Jodocus Greverus, a figure who describes himself as a Presbyterian but about whom almost nothing else is known. The text is preserved in the third volume of the Theatrum Chemicum (pages 699-721 in the 1659 edition), where it appears alongside other alchemical treatises that Maier consulted extensively. Greverus's work is distinguished by its visionary and allegorical character: it describes the author's journey to reach a mystical garden where the philosophical tree grows, a narrative form that connects it to the tradition of alchemical visionary literature extending from the Visio Arislei through the later Rosicrucian allegories.

The heart of Greverus's text is a visionary description of the Garden of the Hesperides, the mythological garden where golden apples grow under the guardianship of a dragon and the three nymph-like Hesperides. Greverus describes a tiring journey to reach this garden, where the philosophical tree is produced by nature, its root being Mercury, its trunk and branches being the successive stages of the alchemical work, and its fruit being the philosopher's stone. The vision serves as an allegorical manual for the entire opus, encoding the stages of the Great Work as stages of a difficult pilgrimage.

De Jong identifies Greverus alongside Robertus Vallensis as a direct source for the frontispiece commentary of the Atalanta Fugiens. The frontispiece depicts the Garden of the Hesperides, the race of Atalanta and Hippomenes, and Hercules gathering the golden apples, all within an ornamental border that frames the title. Together, Vallensis and Greverus establish the precedent for reading the Hesperides-Atalanta mythological complex as an allegory of the volatile-fixed dynamic in the Great Work. Maier drew on both authorities in constructing his frontispiece discourse, and his epigram's reference to hidden treasures within the emblems echoes Greverus's visionary language.

Maier held Greverus in high esteem. De Jong notes that Maier speaks of Greverus and his work with appreciation in the Symbola Aureae Mensae, and that he quotes Greverus in his emblems and discourses throughout the Atalanta Fugiens. The vision of Greverus turns out to be one of the hidden treasures to which Maier refers in his epigram, suggesting that Maier saw Greverus's visionary garden as a key to understanding the entire emblem sequence.

Greverus's description of the philosophical tree, with Mercury as its root, provides a specific source for the tree imagery that recurs throughout the Atalanta Fugiens, including the tree of life in Emblem XXVI and the coral tree in Emblem XXXII. The arboreal symbolism connects the alchemical process to natural growth: the stone grows from its mercurial root as a tree grows from its seed, requiring time, patience, and the right conditions.

For the modern reader, the Secretum Nobilissimorum exemplifies the genre of alchemical vision literature, in which the stages of the Great Work are encoded as stages of a spiritual journey. Greverus's garden is not a physical place but an inner landscape that the alchemist must traverse through the work of the opus. Understanding this genre is essential for reading the Atalanta Fugiens, whose frontispiece garden sets the stage for the entire fifty-emblem journey that follows.