Baptism changed both in its practice and its artistic representation from its introduction in the Gospels to its depictions across centuries of medieval art. The Christian ritual's origins came from both pagan ceremonies and Jewish ones, where the participant is purified by water in order to be suitable for worship or to signal a new phase of one's life. In the Gospels, John, a Jewish rebel leader and prophet, baptizes Jesus, and in so doing endorses him as the Messiah. An important feature of this event is that it was performed in the river Jordan, which was cited in the Torah as the river the Jews crossed to pass from slavery to freedom. Locating the baptism here suggests that Jesus is transformed, in the context of the Gospels, and Judaism, too, is transformed into a new religion. Or, perhaps a medieval Christian viewpoint would be that Judaism was fulfilled in its transformation into Christianity. What we see in many depictions of the baptism of Jesus is that John is not alone in witnessing the transformation--the River Jordan is itself an important character.
Mosaic at the Arian Baptistry, Ravenna
The importance of John the Baptist and the river to the baptism shows up in medieval art countless times. The baptism is John's defining moment in art (superseding his other main stories in Christianity, such as his miraculous birth to his menopausal mother, Elizabeth, and his execution by Herod Antipas as an enemy of Roman rule in Judea).
John's a physically distinctive figure in art: always shown wearing an animal pelt, looking thin and unkempt. He comes into the Gospels from the wilderness, because his prophesies are banished by the established religious centers of Jerusalem. He's the ultimate certifier of Jesus' legitimacy as Messiah before the Crucifixion. The early certification presented in art is probably Jesus' presentation at the temple, where he schools the rabbis. If the rabbis give the certification that comes from the city, with its political/social authority, then John's baptism of Jesus can be seen as coming from the natural (and by extension, I would argue, the pagan) world. John straddles both worlds. He is referred to in Orthodox Christianity as the "Prodomus," or "Forerunner," as the first witness of the descent of divinity into Christ, and as the prophet who foretold his coming. As this forerunner, he engages the pre-Christian landscape as co-witness to the new divinity.
In the photo above, from the dome mosaic in the Arian Baptistry in Ravenna (circa 500), John is muscular in his tattered leopard-skin tunic. He has his right hand on the head of a very youthful-looking Christ. (The beardless Christ may be symbolic of the essential tenet of Arianism, which was that Christ existed after God the Father, and from him. This depiction makes him look more like someone's young son than a mature man. Much ink and blood were spilt stamping out this heresy, to assert the co-eternal nature of the members of the Trinity as orthodoxy.) In John's left hand is a staff that resembles a crozier. Above Christ, the Holy Spirit descends in the form of a dove, accomplishing the spiritual transformation of the baptism. The dove, the river, and John's gesture are all documented in books of the New Testament. What is decidedly NOT in the New Testament is the figure on the left.
He is a personification of the Jordan. To be more precise, he is the river god of the Jordan. (Wikipedia has him listed as Zeus, without citing any academic source for the claim, and I find that to be utterly wrong. He has none of Zeus' attributes, and Zeus' presence would make zero sense in the narrative.) You may, like me, find this an odd choice for representing a defining moment in a monotheism . I've been mulling it over for a couple of years, and reading through whatever I could get my hands on about the baptism of Jesus and river gods in Western art, and I find myself still very much in the land of speculation. You'll have to decide for yourself as I make a case here.
This river god is dressed and positioned in the same way that ones in classical art are: he is bare-chested, half-reclining in the water, and an amphora pours out endlessly the living waters of the river. Usually, the depiction has a cornucopia instead of an amphora, but amphorae are not unheard of. I can't tell what the plant is that he is holding: it could be a reed, or some water plant from the area which I don't know. What it signifies is that from this water flows life. And the river god of the Jordan had domain over it--the plant is wielded like a scepter. He has a nimbus of fire over his head, quite distinct from Christ's halo. The nimbus marks his divinity in a pagan tradition.
What is he doing in this scene of monotheistic consecration? One thing I have seen demonstrated repeatedly through years of studying the development of Christianity in Europe is that true conversion to monotheism in the West took centuries, if not more than a millennium. Early Christian sarcophagi have been found with items associated with other religions (such as the Isis cult, or images of classical deities) from the beginnings to the 600s. In some cases, the symbols and figures of pagan religions were adapted to Christianity (such as the likening of Christ to Dionysus, with vegetation god metaphors). These adaptations helped early converts learn the new religion in terms of worship they already understood. I believe that Ravenna and other parts of Italy still understood the Ancient Greek and Roman animism which would think of the river as an entity. I don't think the Christians who designed the mosaic worshipped rivers, but they had an artistic tradition which depicted rivers as personifications.
Similarly, the artistin Ravenna represented the Apostles surrounding the baptism image dressed like Roman senators (or, more generically, as philosophers--the toga signifying learned, authoritative men). These classical accoutrements lend an air of tradition and prestige to the figures, and to the baptism scene.
The Jordan, in one of the scriptural accountings of the baptism, acts with agency when Jesus is baptized: its flow reverses. It would make artistic sense to see the Jordan as being moved by the Christian god, and that this might be seen as happening by moving the river as one might convert a sentient being. There's other symbolism at work in the changing of the flow of the Jordan in Christian philosophy: the idea of the river's flow from a divine source which descends on the believer, while in baptism the spirit/soul within the believer flows back to its source.
Mosaic at the Neonian Baptistry, Ravenna
Less than a mile away from the Arian Baptistry is the Neonian Baptistry, which is about a century older. Its dome has the same thematic program: the baptism in the center, and the apostles surrounding on the periphery. John's loincloth is a bit more skimpy, and he's built much like Hercules is in classical depictions (compare him to the more lithe Jesus and the less-athletic-looking apostles). He seems to be holding in his left hand a long, jeweled crozier--very Byzantine. He uses a patera to pour the water over Christ's head. He seems here to be more of a natural man than an outcast prophet.
And, look! In the lower-right corner, helpfully labeled, is the river god. His robes seem more like lily pads, and his plant is decidedly a water grass of some sort. He is much less prominently featured, but he's a part of the scene. In both mosaics, I feel that John the Baptist is serving as a priest to the natural world, asserting and officiating at an anointing of a broader, more encompassing divineorder than might be had in a city temple. Before Christ enters Jerusalem (announced as a king), he is consecrated by baptism in nature, outside the city walls. The river god's presence here (akin to, perhaps, the godparents at an ordinary baptism in the tradition this initiates) is a stand-in for both the natural world and the pagan tradition--all of these acknowledging the supremacy of the new religion.
One might chalk up the river god motif in these two baptistries to a very localized phenomenon, but it does seem to persist for another millennium in Italian Christian art.
Parma Baptistry, 14th Century
Inside the baptistry of Parma, the baptism of Jesus by John the Baptist is painted a few times on the walls. Above, you can see a tiny figure at the bottom of the scene, by Jesus' feet. It's crude (this painting is probably the cartoon that survived under the lost original fresco, but it's still discernible as a a river deity--the same pose, the same nudity, and roughly the same place in the water as with the earlier mosaics. Greatly diminished, as were most pagan references at the time. However, it is roughly the same size as the depiction of God the Father and the Holy Spirit placed above John's ministering hand.
Also in the Parma Baptistry, perhaps 13th Century
In this image, opposite the one above it, we see the same river god figure, a bit better-defined. I'm not sure what it's holding in its hand. The compositional logic of both paintings suggests that the small figure is on or under water, the understanding of perspective necessary to paint a figure in water not having been developed yet.
Frieze on lintel above the Parma Baptistry Entrance
We see the perspective problem on the exterior of the building, too (without the river god). I love how the mound-of-water effect is used here, both to convey the river and to use it to conceal Jesus' nudity. The attending angels with elegant bath towels remind me of Boticelli's much later use of the same concept in The Birth of Venus.
The use of river gods in Christian religious art had a resurgence in the Italian Renaissance, where they were used to decorate the Papal Palace and other prestigious religious buildings in Italy. But I thinkthese baptistries showample evidence that the concept didn't die out only to be suddenly revived in the 16th century.
Four Rivers of Paradise, 12th century, Toulouse
This capital from the La Daurade monastery (but now in the Augustinian Museum in Toulouse) depicts the four rivers of Paradise (Tigris, Euphrates, Phison, Gihon) each as a river god, naked and pouring water out of a cornucopia on the corners. The museum curation suggests that in the 12th century the four rivers could also symbolize the four evangelists, but I think that the four rivers theme was more likely a part of a program in the monastery where the End Times were being represented (as was common), with Paradise being a promise of the afterlife. Genesis locates the boundaries of Paradise with these rivers, and, as with the Sea of Glass in the book of Revelation, the river's flow is a boundary between the profane and the sacred.
Where does this leave us with John the Baptist and the River Jordan? He's a figure much like Hermes or other psychopomps. Through baptism, he guides the soul toward redemption and toward its divine source. All the characteristics associated with him in art place him outside of the city (indeed, it's the wrath of Herod in Jerusalem which gets him executed). That he uses the waters of the River Jordan for transformative purposes places him in a cooperative arrangement with the river god, and nature. The rite of baptism transports the soul from the natural realm to the spiritual one, from the pagan to the Christian. John himself performs the act of bestowing divinity upon Jesus.
A syncretism is at work in representing baptism through the character of John. His story in the Gospels takes the fact of his own political clout among Jews who wish to rebel against Roman occupation, and then carefully displays his endorsement of Jesus as the true Messiah, and finallygoes to this singular place, where a ritual is invented (or evolved from Jewish and pagan practices) to initiate believers into Christianity. In this river baptism, his story and importance move from the politics of Jewish rebellion against Rome to casting him as a kind of midwife to the new Messiah. He is no longer "John" in his own right. He is renamed and defined by this single act of divine ritual.
John the Baptist, 14th c. altar cloth, Galleria Accademia, Firenze
In medieval Christianity, John's baptism is also linked to the project of evangelism. He baptizes the apostles after the Crucifixion of Jesus, and he does this, as with Jesus, in the river Jordan. The river's act of flowing outward or through territories corresponds effectively with how the apostles are charged to flow out from Jerusalem to convert people everywhere to Christianity.
In the 10th century in Constantinople, a relic of John's right hand, which brought the waters of the Jordan to touch the top of Jesus' head, was used to consecrate kings (his hand touching their heads). His act of announcing Jesus as king through baptism is brought back to the medieval political world in triumphant Christianity, now a religion spread across continents. With the character of John, one can see the Christian project turning from its early focus on fulfilling Judaism to becoming a separate religion: a religion with the intention of encompassing the entire world.
In medieval art, John and the river god provide a necessary witnessing component to representing the birth of the Church. Their figures preside over the replication of the rite of conversion, which is acted out in the Middle Ages by the officiating priest, the father, and the godparents. (Mothers couldn't attend baptisms while they were deemed ritually impure by the act of childbirth.) In this sense, John and the river god are priest and godfather to the new religion. I'm struck by the complete masculinity of this scene, and ritual. Divinity, and even the natural world, have no feminine qualities in these depictions. The baptism rite is a reinforcement of the lines of spiritual authority, which by the Middle Ages had far fewer roles for women than early Christianity.