In Angels & Demons the Path of Illumination is a trail of four locations called the Altars of Science, containing works of art which point to the Illuminati Lair, the meeting place of the ancient society.
To get the path, Langdon first has to solve a poem written in English by John Milton and placed in Galileo's Diagramma della Vertia. The poem reads:
From Santi’s earthly tomb with demon’s hole,
’Cross Rome the mystic elements unfold,
The path of light is laid, the sacred test,
Let angels guide you on your lofty quest.
People who wished to be members of the Illuminati would have to know what this poem referred to. Santi, is Raphael Santi and his tomb is not his own in the Pantheon, but one he designed in Santa Maria del Popolo. 'Cross Rome literally means across Rome but also as the location of each altar forms the shape of a cross on the map. The path of light is the path of illumination, where at each altar an angel statue will point to the next altar.
The Fourth marker will point to the Church of Illumination in Rome, the secret meeting place of the Illuminati.
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