The Pantheon has been called a “perfect” space because the diameter of the rotunda is equal to that of its height (43m, 142ft). The purpose of this space was to suggest geometrical perfection and symmetry in the context of a perfect universe. The interior space could fit perfectly either in a cube or in a sphere. The massive interior room is designed to symbolize the heavens; the oculus or Great Eye in the room is designed to symbolize the light- and life-giving sun.
Oculus of the Pantheon:

The central point of the Pantheon is far above visitors’ heads: the great eye, or oculus, in the room. It looks small, but it’s 27ft across and the source of all light in the building — symbolic of how the sun is the source of all light on earth. Rain that comes through collects in a drain in the center of the floor; the stone and moisture keep the interior cool through the summer. Every year, on June 21st, the rays of the sun at the summer equinox shines from the oculus through the front door.
Construction of the Pantheon:

How the dome has been able to bear its own weight has been a matter of great debate — if such a structure were built today with unreinforced concrete, it would quickly collapse. The Pantheon, though, has stood for centuries. No agreed-upon answers to this mystery exist, but speculation includes both an unknown formulation for the concrete as well as spending a lot of time tamping the wet concrete to eliminate air bubbles.
Changes in the Pantheon:

Some lament the architectural incoherence in the Pantheon. We see, for example, a Greek-style colonnade on the front with a Roman-style interior space. What we see, however, is not how the Pantheon was originally constructed. One of the most significant changes was the addition of two bell towers by Bernini. Called “asses’ ears” by Romans, they were removed in 1883. In a further act of vandalism, Pope Urban VIII had the bronze ceiling of the portico melted down for St. Peter’s portico.
Pantheon as a Christian Church:

One reason why the Pantheon has survived in such remarkable shape while other structures are gone may be the fact that Pope Boniface IVI consecrated it as a church dedicated to Mary and the Martyr Saints in 609. This is the official name which it continues to bear today and masses are still celebrated here. The Pantheon has also been used as a tomb: among those buried here are the painter Raphael, the first two kings, and first queen of Italy. Monarchists maintain a vigil at these latter tombs.
Influence of the Pantheon:

As one of the best surviving structures from ancient Rome, the influence of the Pantheon on modern architecture almost cannot be underestimated. Architects from all over Europe and America from the Renaissance through the 19th century studied it and incorporated what they learned into their own work. Echoes of the Pantheon can be found in numerous public structures: libraries, universities, Thomas Jefferson’s Rotunda, and more.
It’s also possible that the Pantheon has had an impact on Western religion: the Pantheon appears to be the first temple built with general public access in mind. Temples of the ancient world were generally limited only to specific priests; the public may have taken part in religious rituals in some fashion, but mostly as observers and outside the temple. The Pantheon, however, existed for all the people — a feature which is now standard for houses of worship in all religions of the West.

Hadrian wrote about the Pantheon: “My intentions had been that this sanctuary of All Gods should reproduce the likeness of the terrestrial globe and of the stellar sphere...The cupola...revealed the sky through a great hole at the center, showing alternately dark and blue. This temple, both open and mysteriously enclosed, was conceived as a solar quadrant. The hours would make their round on that caissoned ceiling so carefully polished by Greek artisans; the disk of daylight would rest suspended there like a shield of gold; rain would form its clear pool on the pavement below, prayers would rise like smoke toward that void where we place the gods.”