Ogham was invented by the first Christian communities in early Ireland,
The Ancient Celts were not an illiterate people, but they transferred their knowledge orally.
They had an alphabet of twenty letters called Ogham.
Each letter was named after a tree from the land where they lived.
Ogham was used on standing stones, primarily on graves and boundary markers
. The primary sources of information about the Celts are, in that light, the texts written by the Romans who were in touch with them and Christian monks, who lived in Irish monasteries in the Middle Ages.
Caesar, Livy and Tacitus, wrote about their contemporaries who lived in a way different than themselves and therefore were considered barbarians, but even though they did not have a positive attitude towards them, they still left some useful information about Celtic society, religion, way of life, and so on.
One of the problems that arise from this is that many things in these writings are romanised, e.g. Caesar interprets Celtic gods and calls them by the names of their Roman equivalents
They worship as their divinity, Mercury, in particular, and have many images of him, and regard him as the inventor of all arts, they consider him the guide of their journeys and marches, and believe him to have great influence over the acquisition of gain and mercantile transactions.
Next to him they worship Apollo, and Mars, and Jupiter, and Minerva respecting these deities they have for the most part the same belief as other nations that Apollo averts diseases, that Minerva imparts the invention of manufactures, that Jupiter possesses the sovereignty of the heavenly powers that Mars presides over wars.
The second type of sources are the books written from 6th - 13th century on by Christian monks in Ireland and Celtic Britain.
These books were written several centuries later, so the oral tradition might have changed and much of the information was under Christian influences. The Celts were one of the most significant and powerful peoples in Europe