What is the underlying meaning of Foucault's Pendulum?

The chief underlying meaning of "Foucault's Pendulum" is the relatedness of seemingly unrelated ideas. Casaubon's research begins as a casual interest in seemingly separate secret organizations, such as the Knights Templar, which spirals into an obsession when he unravels one thread only to discover that it is woven into a larger tapestry than any historian or archaeologist has yet to imagine. As Casaubon progresses further in his research, the connectedness he observes becomes more overt. A subtle theme of the work is whether or not this connectedness is an element of objective reality or rather inspired by Casaubon's own obsession.
Other characters, from Casaubon's colleague, Jacopo Belbo, to his largely atheistic girlfriend, Lia, raise the question of Casaubon's perceived connections in subtle ways throughout the story. While Casaubon becomes prone to observing connections between ancient societies and rituals, Lia remains grounded in objective reality, providing a much-needed counterweight of rationalism.
The theme of connectedness reaches its zenith when Casaubon and his colleagues discover a formula woven throughout human history. The realization of a connectedness beyond his ability to imagine sends editor Diotallevi into isolation and the brink of madness, draining his very life force. The connectedness uncovered by Diotallevi, Casaubon and Belbo begins to take on a cosmic nature. While Diotallevi loses himself to his obsession, Casaubon takes a more playful attitude towards their discovery. In this way, "Focault's Pendulum" examines the theme of interconnected truth through the very different perspectives of its three major characters. To Diotallevi, the connectedness itself takes on godlike qualities, while to Casaubon and Lisa it remains a fascinating yet detached concept to study.