Pre-Roman Rituals in the Belgic regions of Gaul (Gallia Belgica) are hard to extrapolate without considerable work. The seeming illiteracy of the Celtic peoples in Belgic Gaul often hinders efforts to learn about the indigenous religions that varied from region to region, unless we consider that the Roman presence actually encouraged such practices. It’s entirely possible that it wasn’t a complete override of the previous religions and that the Belgic peoples adopted the ‘Roman’ ways on their own. This is evidenced in the newly published book Celtic Religions in the Roman Period, edited by Ralph Haeussler and Anthony King. 
If this is true, then perhaps we don’t necessarily have to cling on to a too romantic notion of ‘Celtic’ pride where everything Latin or Roman in origin must be thrown out in order to truly be Celtic or Belgic, and therefore functional. In order to figure out ritual format, we must look at the various sanctuaries the Beligic people had. Jean Louis Brunaux in his book The Celtic Gauls: Gods, Rites, and Sanctuaries lists four types of sanctuaries, Belgic, Viereckschanzen (quadrangular enclosures), Celto-Ligurian, and Spring Sanctuaries. The Belgic sanctuaries listed in the book are very striking representatives found among the regions of the Bellovaci and Ambiani. 
The first example he lays out is from the Sanctuary at Gournay, which is dated at least back to 4th Century BC to 4th Century AD. Characteristically, these sanctuaries contain offerings of remains of sacrifice, such as ritually broken objects, bones, and plants. The next characterisitic is that there is an enclosure around them, a ditch and a palisade which separates the sacred space from the profane. The last characteristic is the actual temple or a group of posts. These sanctuaries are often found on plateaux and on an edge or highest point. Brunaux makes a point to say that it’s always in a dominant position. This information will be handy should we decide in the future to formulate plans on how to create a Belgic styled sanctuary. However, as we noted above, there were types of offerings that can be identified: objects of various nature (pots for example), plants (which seem to be the most common example), animal sacrifice and also libations (which can fall under plants, such as beer or mead or whine), as well as temple and statue offerings.
The two categories of plant offerings were freshly harvested cereals, fruits or branches, and loaves, cakes, crushed cereals and beverages (I suppose we can say harvested or ‘processed’ plants. Not necessarily over processed as in a factory setting, but in fact plants that were turned into other food stuff). Therefore we have our first characteristic of Belgic styled ritual and practice: Offering of some sort of plant in harvested or processed form.
‘Caesar’s evidence is explicit: ‘When they have decided to fight a battle they generally vow to Mars the booty they hope to take, and after a victory they sacrifice the captured animals and collect the rest of the spoil in one spot’. The Celtic counterpart to Mars was not the only beneficiary. Among the Insubres he shared the perogative with Vulcan: Ariovistus was busy consecrating a torque of gold to the first while Viridomarus consecrated all the arms that he had taken to the second.’
This evidences that the vow practice was known among the Gaulish tribes even before cultural adaptation, even if plenty of offerings and votum found in excavations were largely Roman in nature. It could mean that this was a fairly easy transition into Roman practice for Belgic peoples, and therefore Belgic religions. If this is the case, we have our theoretical ritual format that counts as both Celtic and Roman: the Votum.
H.S. Versnel defines Votum as such:
‘A vow. Both Greeks and Romans habitually made promises to gods, in order to persuade them to grant a favour stipulated in advance. If the gods fulfilled their part, the vow-maker fell under the obligation to do as he had promised. Although the practice was no less popular in Greece, the vow developed an institutional form especially in Rome, owing to the practical and juridical aspect of Roman religion. Expressions such as v(otum) s(oluit) l(ibens) m(erito) (‘NN has paid his vow with pleasure and deservedly’), mainly in private votive gifts, and voti reus, voti damnatus (‘obliged to fulfil his vow’), mainly in public vows, belong to the fixed formulas. In the private sphere prayers for recovery and good health, crops, childbirth, safe return from an expedition, etc. were, in case of fulfilment, answered by a great variety of votive offerings. In public votive religion it was the magistrate who in the name of the state undertook to offer to a god or gods sacrifices, games, the building of a temple or an altar etc. , if the god on his side would give his assistance in such basic collective crises as war, epidemics, and drought. Formulas had to be pronounced in public and were very strict: mistakes required the repetition of the whole ceremony. In addition to these extraordinary vows there were also regular vota, pronounced for a definite period: e.g. the annually renewed vota of the magistrates for the welfare of the state on 1 January before the first regular sitting of the senate, and the vota at the termination of the lustrum. Such vows found their direct continuation under the empire in the vota pro salute imperatoris (for the health or safety of emperor and his family) and became periodical: vota quinquennalia, decennalia (for five, ten years). Extraordinary vows (for the safe return of the emperor from an expedition, for the recovery of the empress in cases of sudden illness) continued to exist into late antiquity. The text of the votum was officially fixed in the presence of the pontifices, and the document went into the archives.’
In this, we are concerned with personal religion styled to Belgic religion, therefore we will focus on private votum practice. Versnal also writes:
‘Let us, for the sake of convenience, recall C. Ausfeld’s formal division of prayer into invocatio (invocation by means of the name, surname, epithets and descriptive predicates), the pars epica (in which the suppliant explains why he is calling on this particular god for help, what his relation- ship with the deity is, and why he thinks he can count on his assistance) and the actual preces (the content of the wish).’
We then have three steps for prayer:
2. Pars Epica
In terms of personal religion, one would perform the tripartite prayer format. Then after one would receive the god’s gift, the performer would give ex voto (the actual offering) along with making documentation, proving that the vow was fulfilled, keeping the documentation in a separate space dedicated for such a practice. As Versnel marks, public Votum was kept by Pontifices in archival storage. However, inscriptions do show private votum using formulaic phrases, so it’s not a stretch to say that personal religion can in fact do the same.
Thus, for our purposes, we would have a quadripartite prayer format for Votum or Oitos:
1. Invocatio / Uediiâ (Invocation):
2. Pars Epica / Breitra (argument):
3. Preces/ Petiûmi (Petition/Prayer):
4. Votum / Oitos (Oath/Vow):
If one wanted to, they could take out the Roman element of documentation of the votum and leave the entire process oral, as pre-Roman practices indicate. However, considering the readiness to adapt said Roman practices, it would not be an off color addition to the ritual format. If it’s desired to be kept, we can make it more ‘Celtic’ by utilizing Gaulish dialects in the documentation and use characters other than the Latin Alphabet, such as the Lepontic Alphabet, which would be appropriate considering the ancestral Belgic rulers migrated from the Balkans, where it was used. This wouldn’t be a requirement, but it would a little more flavor to the praxis.
An example then would be:
Invocatio / Uediiâ (Invocation):
Gaulish- Sironin wediûmi, Dêwin Lugrâs, Dêwin Admesserâs, Ariyin Natrigon, Ariyin Andounnânon.
English- Sironâ I invoke, The Goddess of the Moon, The Goddess of Time, Lady of Serpents, Lady of Wells
Pars Epica / Breitra (argument):
I don’t have an example in Gaulish for this just yet, but Versnel explains ‘in which the suppliant explains why he is calling on this particular god for help, what his relation- ship with the deity is, and why he thinks he can count on his assistance’. So in English, it might be something akin to ‘I call to you because you, night serpent, have never abandoned me and have always kept me and mine in good health”. Or even a recounting of the god’s deeds mythologically.
Preces/ Petiûmi (Petition/Prayer):
Gaulish- Yâ detsi slaniyin amê eti wirobo anextlon bouboc.
English- That she give health/safety to us and protection to people and cattle.
Votum / Oitos (Oath/Vow):
This is where one would make a vow to the god in question. In English, this might look like ‘I promise to sacrifice 5 loaves of bread in your name in return for keeping my health and cattle safe’. It could even be something more devotional, such as ‘I promise to offer one stick of incense daily without fail for three hundred sixty five days’.
It would then be up to the petitioner to receive the gift of the god, and follow through with the ex voto (ie offering mentioned in the votum). As was mentioned, it can be documented for records of piety and placed in a sacred/separate space, possibly with the Latin Alphabet, English, or Lepontic Alphabet.
Like in Latin, the votum inscription can be abbreviated. The phrasing of v(otum) s(oluit) l(ibens) m(erito), which is one of the more common formulas in votum, in a Gaulish dialect might look like X uregât eion(masculine)/eiâs(feminine) oiton duci budî eti uîrê – X did his vow by will and truth.
Example: How it might look like documented in the Lepontic alphabet:
To Sironâ, Selgowiros did his vow by will and truth.
The abbreviation for UOBU is on the bottom of the example for standard use, should the practitioner decide to use documentation. The system for abbreviation for personal names and gods will probably have to be worked out between the writer and dedicant.
1. Celtic Religions in The Roman Period: Personal, Local, and Global edited by Ralph Haeussler & Anthony King 2018 (entry: Celtic Goddesses From Gallia Belgica and The Germaniae: Characteristics, Dedicants and Ritual Practices by Audrey Ferlut) P. 374-375
2. The Celtic Gauls: Gods, Rites, and Sanctuaries 1987 by Jean Louis Brunaux P. 11
3. The Celtic Gauls: Gods, Rites, and Sanctuaries 1987 by Jean Louis Brunaux P. 12-13
4. Les Religions Gauloises, aouvelles approches Su Les Rituels Celtiques de la Gaule indépendante 2000 by Jean Louis Brunaux P. 147-9
5. The Celtic Gauls: Gods, Rites, and Sanctuaries 1987 by Jean Louis Brunaux P. 103
6. Celtic Religions in The Roman Period: Personal, Local, and Global edited by Ralph Haeussler & Anthony King 2018 (entry: Celtic Goddesses From Gallia Belgica and The Germaniae: Characteristics, Dedicants and Ritual Practices by Audrey Ferlut) P. 377
7. Votum definition on Oxford Classical Dictionary online by H. S. Versnel http://classics.oxfordre.com/view/10.1093/acrefore/9780199381135.001.0001/acrefore-9780199381135-e-6862?result=194&rskey=ixzUuP
8. Faith, Hope and Worship: Aspects of Religious Mentality in the Ancient World. Studies in Greek and Roman Religion Volume 2 edited by H. S. Versnel in Co-operation with F.T. Van Straten 1981 P. 2