But Jefferson's point was that Christianity itself was never made a part of the common law.
And Jefferson himself says that Christianity arrived around the year 600. In this he is not, strictly speaking, correct.
Quoting from an article on the history of Christianity in England:
The first evidence of Christianity in England is from the late 2nd century AD. (There may have been Christians in Britain before then, we cannot be sure). Roman Britain was a cosmopolitan place. Merchants from all over the empire settled there and soldiers from many countries served there so we will never know who first introduced Christianity to England.
At that time England and Wales were ruled by the Romans. The native people were Celts. They were polytheists (they worshipped many gods). The Romans too were polytheists and they were willing to allow the Celts to worship their old gods.
However the Romans were not tolerant of Christianity. At times waves of persecution crossed the empire. St Alban the first British Christian martyr was executed in a town called Verulamium in 304 AD. Much later an abbey was built there dedicated to St Alban and it gave its name to the town of St Albans.
In 313 the Emperor Constantine granted Christians freedom of worship. So persecution ended and during the 4th century Christianity became widespread in England.
In 314 three British bishops attended a church council in Arles in France, Eborius bishop of York, Restitutus bishop of London and Adelius bishop of Caerleon (Gwent). So by that time [314 AD] there was a flourishing and organised church in England.
In Hinton St Mary, Dorset a 4th century mosaic was found with the face of Jesus and the Greek letters chi rho, which stand for christos (Greek for Christ) showing Christianity was a popular religion in England. [again, in the 300s.]
Christianity in Anglo Saxon England
In 407 the last Roman soldiers left Britain. Over the following decades Roman civilisation broke down. In the 5th and 6th centuries Saxons, Angles and Jutes from Germany and Denmark invaded southern and eastern England and gradually conquered most of England.
However Christianity continued to thrive in Wales and by the early 5th century it spread to Ireland. In the 5th and 6th centuries Scotland was converted. Cut off from the Church in Rome Celtic Christians formed a distinctive Celtic Church.
According to tradition Pope Gregory saw boys on sale in the slave market in Rome. He is supposed to have asked about them and when told that they were Angles he replied ‘not Angles but angels' When he became Pope he was keen to convert the Anglo-Saxons. In 596 he sent a party of about 40 men led by Augustine to Kent. They arrived in 597.
Aethelberht permitted the monks to preach in Kent and in time he was converted. (The king of Kent was married to a Christian princess named Berta. It may have been partly due to her influence that Kent was converted to Christianity). Furthermore his nephew, Saeberht, the king of Essex was also converted.
Meanwhile in 627 King Edwin of Northumbria (the North of England) and all his nobles were baptised. (He may have been influenced by his wife, Ethelburgh, who was a Christian). Most of his subjects followed.
Even if Jefferson were correct about the date Christianity arrived in England, the common law most certainly continued to evolve LONG after the 600s. It was not simply complete and set in stone from the time that the Sacons arrived (which Jefferson puts as being around 450 AD).
In fact, Encyclopedia Brittanica (which seems a fairly authoritative source for things Brittanic) says that the English common law system really came about in the Middle Ages:
"The common-law system originated in England in the Middle Ages."
Wikipedia sheds more light on how this came about:
Medieval English common law
In the late 800s, Alfred the Great assembled the Doom book (not to be confused with the more-famous Domesday Book from 200 years later), which collected the existing laws of Kent, Wessex, and Mercia, and attempted to blend in the Mosaic code, Christian principles, and Germanic customs dating as far as the fifth century.
Before the Norman conquest in 1066, justice was administered primarily by what is today known as the county courts (the modern "counties" were referred to as "Shires" in pre-Norman times), presided by the diocesan bishop and the sheriff, exercising both ecclesiastical and civil jurisdiction. Trial by jury began in these courts.
In 1154, Henry II became the first Plantagenet king. Among many achievements, Henry institutionalized common law by creating a unified system of law "common" to the country through incorporating and elevating local custom to the national, ending local control and peculiarities, eliminating arbitrary remedies and reinstating a jury system – citizens sworn on oath to investigate reliable criminal accusations and civil claims. The jury reached its verdict through evaluating common local knowledge, not necessarily through the presentation of evidence, a distinguishing factor from today's civil and criminal court systems.