The pieve takes the austere composure of its façade and exterior inside as well: no frescos or mosaics interrupt the uniform grey of the stone, immobile in the light of the passing hours of the day. Its aspect today is very similar to the original Romanesque one. A few additions and the total replacement of the pavement during the restorations of 1968-1971 show the care taken over the centuries to preserve the church.
The interior is divided into seven bays with round arches supported by robust monolithic columns, and, between the fifth and sixth bay, by two square pilasters. In addition, there are two embedded pilasters behind the façade, which correspond to two embedded columns on either side of the apse. Two other embedded columns face the columns of the last bay, which, together with two embedded corner columns, enclose the space of the two lateral altars. The sacristy and bell tower are to the right. The church has a truss ceiling, except for the cross-vaulted bays near the apse, where the two lateral altars are. The one on the left is dedicated to the Sacred Heart, the other to Holy Mary, and the main altar to Saints Peter and Paul. The arches of these two bays also have smaller spans than the others, since they are narrower. The late-Lombard pulpit was dismantled, reassembled and remounted on the fourth column on the right by the builders of the Romanesque structure. Gropina is not the only place in Tuscany where one finds stylistically Lombard elements reused in Romanesque churches: San Cassiano in Bagni di Lucca, for example, built in the 12th century, includes some of its pre-existing architectural furnishings.
The capitals in each nave will be described in a separate section.
Two elegant arcades, one over the other, with round arches on thin columns partially imitating and completing those of the exterior, decorate the interior of the apse. Their capitals present acanthus leaves, other floral motifs and volutes, and some contain small shields of a type called “old French” in heraldry, used in the 12th century. Since the construction of a church begins from the apse and proceeds towards the façade, this detail helps date the initial phases of work to that period. The two colonnades below the half dome correspond to the seven large blind arches on the outside of the apse, while the pendant of this calotte is in the exterior loggia, along with the group of two knotted columns in the middle of the other twelve small ones.
The double symbolism of the numbers six and seven, mentioned with regard to the exterior, return inside as well. Six small oculi in the lower order and three rather narrow splayed monophoras in the upper row light the area of the presbytery. What little light they do let in is of a particular kind, which is also highly symbolic: the openings are not protected by plates of glass, but by alabaster cut so thin as to be translucent. This is not the only classical feature here in Gropina that assumed a Christian meaning, as in other cases already mentioned or to be noted later. In the classical world alabaster was in fact considered a divine stone because when cut into thin plates, unlike other types of stone, instead of absorbing light it passes and diffuses it. Alexander the Great was buried in a tomb of alabaster in the city he founded because the oracle deemed him the “son of Amon”, the light of the world, who was therefore expected to shine after his death as in his life. After the advent of Christianity, when the myth of Alexander of Macedonia was clothed in oblivion, alabaster came to represent a new light in the world, referring to the single figure who had succeeded in eclipsing the fame of that great military leader: Jesus Christ. This explains the use of alabaster, the stone of the gods transformed into the stone of God, in the pieve of Gropina: every day at dawn the sunlight enters from the apse, oriented toward the east, slowly dispelling the darkness of the church, like the new sun, Christ, who through his word brings light to the world. The orientation of the pieve along an east-west axis is not unique to Gropina, and was common in many other Romanesque churches for this reason, and in a sense completely opposed to that of the Egyptian temples of Amon, for instance, where Amon, the sun, entered at dawn through the entrance of the temple, proceeding towards the darkness of the naos, or sancta sanctorum, the place shrouded in darkness where the statue of the god dwelled and where only the priests could enter.
The zone of the presbytery of the pieve holds a small treasure having a certain artistic and historic charm, and often overlooked by scholars: this is the stone tabernacle embedded in the Epistle side of the large arch of the apse, on the right. In the form of a small temple, it is decorated by two bands with stylized leaves and curls above and below a wooden door flanked by two pilasters whose capitals are decorated by plant motifs; the tympanum, with its very pointed spire, shows a candelabra, a design imitating an aloe flower, or else a candelabrum with festoons, garlands or other symmetrically arranged elements; in the corbel and on the pilasters there are three Medici heraldic emblems, a bucranium (ox skull) or a horse's head, surmounted by the galero (“ecclesiastical hat”); the elegant shield with bucranium was especially used from the 1470s up to the end of the century, and even more widespread from circa 1480. This and the presence of the galero, allow the tabernacle to be attributed with certainty to Giovanni de' Medici (1475-1521), son of Lorenzo il Magnifico and elected cardinal March 8, 1489. Even if the tassles of the galero are not shown, the emblems certainly refer to Giovanni’s cardinalcy because he became a bishop on March 13, 1513, six days after election to the papacy (until then, in fact, he was only a deacon) and the coat of arms, along with the style of the tabernacle, is clearly from the 15th-century. Therefore the tabernacle can be dated to not before 1489 and possibly to 1492, because Giovanni de' Medici was made cardinal with the prohibition to don the robes of his rank for three years, because of his young age (he was only 14, having been born in 1475). The tabernacle is therefore an example of art of the late Laurentian period, and shows once again the connection that the pieve of Gropina had with Lorenzo il Magnifico and with Pope Leo X.