No other of the world’s great churches has had more capacity to surprise than the basilica of San Clemente in Rome. It was already known for its elegance and beauty, above all for its apse mosaic (Figure 1), which dates from the early twelfth century. Excavations beginning in 1857 revealed first a lower basilica adorned with magnificent frescoes from the eighth to eleventh centuries, and then a level of Roman streets below that, including a well preserved temple of Mithras. Archaeologists continue to uncover its hidden treasures, not least a recently discovered sixth century baptistry.

Figure 1.

The apse mosaic of San Clemente (photograph: Fr Lawrence Lew OP).

The apse mosaic is a work of unsurpassed skill and beauty, its carefully structured imagery still yielding fresh details and greater depths of meaning to the inquiring eye. Some of its tiny images – a couple of snails for example – will have been invisible to anyone standing beneath them until modern lenses brought them into focus. New theological connections have also been identified by sharp-eyed scholars. For example, the influence on the design of St Ambrose’s writings on Genesis was suggested by the solid red halo that distinguishes his image in the mosaic from the other three Doctors of the Church depicted there. This suggestion has been corroborated by a careful reading of the relevant texts.

The outstanding feature of the mosaic’s design, as is well known, is a huge plant, which combines features of a vine and of an acanthus. Four rivers flow from beneath it, a crucifix rises out of its stem, and it branches out into a vast whirling network of fronds that cover the apse and encircle images that represent every class of Christian and every kind of creature. As G.K. Chesterton put it:

Beneath this image is a double inscription, written in elegant Latin (Figure 2). Like the other non-scriptural inscriptions in the mosaic, it is in metrical, indeed mostly rhyming, verse (‘Leonine hexameters’, if you want the technical term), a detail that echoes the eleventh century frescoes in the Church below.Footnote ² One of these two sets of verses contains a new surprise. The relevant parts read:

ECCLESIAM CRISTI VITI SIMILABIMUS ISTI

QUAM LEX ARENTEM, SET CRUS FACIT ESSE VIRENTEM

We compare the Church of Christ to this vine,

Which the LEX makes dry out, but the CRUS makes verdant.

Figure 2.

The inscription below the mosaic (photograph: Fr Lawrence Lew OP).

Virentem, from vireo, is a verb used primarily of plants, evoking green growth. What are the lex and the crus that so affect the Church, and for that matter, the vine? Interpreters have normally jumped to the conclusion that crus, translated and sometimes even transliterated as if it were crux, means ‘cross’. The Cross is, after all, the central image in the mosaic. The contrast then might be between the lex or Law of the Old Testament and the Cross of Christ. We might think of St Paul: ‘we preach Christ crucified, a stumbling block to the Jews’ (I Corinthians 1.23).

There is one big problem with this interpretation: the makers of the inscription did not write CRUX, but CRUS. This was certainly not carelessness on their part: all the inscriptions in the mosaic are impeccably written, including, for example, the use of a range of precise and accurate abbreviations. The designers intended us to read crus. The most familiar meaning of this word is ‘leg’, but it has a secondary meaning common enough in the right context – and this is very much the right context. The crus of a plant is its stem, or rootstock. The word is used in this sense a dozen times, for example, in Albert the Great’s treatise on plants.Footnote ³

The inscription is telling us, then, that just as a rootstock gives life to a vine, so Christ gives life to the Church. The text we immediately recall is John 15.4–6:

The primary theological point is not that the Cross, as an external agent, brings life where the Law has failed, but that we, the Church, draw life from Christ as living parts of a single organism, enlivened, we might say, by the sap that circulates through the whole. Perhaps there is a link here with another image in the mosaic, that of St Peter saying to St Clement (also in Leonine hexameters!):

We are rooted in Christ, this might suggest, through the inherited authority of the Church.

At the same time, the designers of this intelligent mosaic must have intended a rich pun: crus and crux are intimately connected, exactly as the image displays them. The power of the Cross flows from the power of Christ, the rootstock. We are reminded that the Cross was itself once a living plant, a tree that was linked with the burial place of Adam, and through him to the garden of Eden, symoblised in the mosaic by the serpent and the four rivers, both near the foot of the acanthus/vine.

What then is the lex? It looks as if a double comparison between the Church and the vine is intended. Just as the rootstock/Christ gives life to both vine and Church, so the lex makes each of them wither. How on earth can a law make a vine wither? I am hoping that some expert on medieval viticulture might come forward with a better suggestion, but for now the following is the best I can do. One sense of lex given by the Oxford Latin Dictionary is ‘a principle inherent in the nature of a thing, a condition of existence’. Thus the ‘lex’ of a vine would include its being deciduous. To quote the first letter of St Clement, which is likely to have been known by the mosaic’s designers, ‘Take a vine: first it sheds its leaves, then comes a bud, then a leaf, then a flower, after this the unripe grape, then the full bunch’ (XXIII.4). To medieval people familiar with growing grapes, this ‘law’ of the vine’s existence would have been striking. Just before they begin to bud, vine stems look very very dead – long, dry and utterly withered (I write this as the vine in our own greenhouse is in exactly this condition). Its bursting into new life, a life hidden away all winter in its crus, seems like a yearly miracle.

If this is right, then the mosaic evokes more than a one-off event that brings the Church to life, a point that perhaps fits well with the present tense of facit, ‘makes’. It seems to suggest regular renewal through the life-giving connection with our ‘stem’. Even when the Church seems at its most lifeless, the imagery suggests, we can find new life in Christ.

Finally, what of lex in relation to the Church? At this point, we are greatly helped by the mosaic’s most recent scholarly interpreter, Stefano Riccioni. In his monograph, Il mosaico absidale di S. Clemente a Roma: ‘Exemplum’ della chiesa riformata, he locates the creation of the mosaic in the context of the Gregorian Reform. He points out that the architecture of the basilica made a clear separation between clergy and laity, and that only the clergy would have been able to read this inscription: it was meant for the theologically and ecclesially literate. On the other hand, precisely because of their learning, its viewers would have been able to interpret it in rich and diverse ways, including noticing the interplay between images and texts.

Specifically, Riccioni identifies three possible contexts for elucidating the text. The first was a widespread theological debate, stimulated in part by the Crusades, with and about the Jews. (Thankfully, in Rome, unlike some other places, there were no manifestations of aggressive anti-Semitism.) New reformed Bibles were produced that highlighted the role of the prophets in interpreting the Old Testament as foreshadowing Christ. The two prophets depicted besides the apse, Isaiah and Jeremiah, Riccioni notes, also compared Israel to a degenerate vine. A second context was the replacement of the old version of the Rule (or lex) of the Canons Regular in San Clemente, as part of the Reform, which emphasised an accompanying need for conversion of life. The third context was the argument about spheres of authority between the emperor and the pope. Specifically, Pope Paschal II, a former Cardinal-priest of San Clemente, had, when imprisoned by Henry V, given in to his demands for the right to appoint prelates. The law (lex) that permitted this was seen by some reformers as a betrayal.

Riccioni’s generous approach to interpretation reminds us of the inexhaustible wealth of this mosaic: there is always something new to see, some new connection to make. Each of his suggested contexts remind us of the reforming theme of conversion, rooted in the Church. It is the spirit, or Spirit, not the letter, of any reform that gives it life (cf. 2 Corinthians 3.6). The little word crus has power here. The only source of any renewal within the Church, one available not once only but repeatedly, is close union with the stem that is Christ.

The fifty vine scrolls that curl across the apse represent, it is argued, the fifty years of Jubilee: ‘You will declare this fiftieth year to be sacred, and proclaim the liberation of all the inhabitants of the land’ (Leviticus 25). The text tells us that the vineyards would be blessed in the previous harvest so that they produced threefold, allowing the vines to be left to rest in the Jubilee year. This reference would form part, Fr Paul Murray suggests, of the mosaic’s ‘core message of joy and jubilation’. The highlighting of the rootstock of the vine, able repeatedly to restore life to the most withered of plants, is well suited to this sense of hope. In our own Jubilee Year of 2025, one modest ‘S’, so often overlooked, allows the basilica to offer us one more small surprise.