The native Britons had converted to Christianity under Roman rule.
The Anglo-Saxon invasions separated the British church from European Christianity for centuries, so the church in Rome had no presence or authority in Britain, and in fact, Rome knew so little about the British church that it was unaware of any schism in customs.
However, Æthelberht would have known something about the Roman church from his Frankish wife, Bertha, who had brought a bishop, Liudhard, with her across the Channel, and for whom Æthelberht built a chapel, St Martin's.

In 596, Pope Gregory the Great sent Augustine, prior of the monastery of St. Andrew in Rome, to England as a missionary, and in 597, a group of nearly forty monks, led by Augustine, landed on the Isle of Thanet in Kent.
According to Bede, Æthelberht was sufficiently distrustful of the newcomers to insist on meeting them under the open sky, to prevent them from performing sorcery.
The monks impressed Æthelberht, but he was not converted immediately.
He agreed to allow the mission to settle in Canterbury and permitted them to preach.

It is not known when Æthelberht became a Christian.
It is possible, despite Bede's account, that he already was a Christian before Augustine's mission arrived.
It is likely that Liudhard and Bertha pressed Æthelberht to consider becoming a Christian before the arrival of the mission, and it is also likely that a condition of Æthelberht's marriage to Bertha was that Æthelberht would consider conversion.
Conversion via the influence of the Frankish court would have been seen as an explicit recognition of Frankish overlordship, however, so it is possible that Æthelberht's delay of his conversion until it could be accomplished via Roman influence might have been an assertion of independence from Frankish control.
It also has been argued that Augustine's hesitation—he turned back to Rome, asking to be released from the mission—is an indication that Æthelberht was a pagan at the time Augustine was sent.

At the latest, Æthelberht must have converted before 601, since that year Gregory wrote to him as a Christian king.
An old tradition records that Æthelberht converted on 1 June, in the summer of the year that Augustine arrived.
Through Æthelberht's influence Sæberht, king of Essex, also was converted, but there were limits to the effectiveness of the mission.
The entire Kentish court did not convert: Eadbald, Æthelberht's son and heir, was a pagan at his accession.
Rædwald, king of East Anglia, was only partly converted (apparently while at Æthelberht's court) and retained a pagan shrine next to the new Christian altar.
Augustine also was unsuccessful in gaining the allegiance of the British clergy.
The native Britons had converted to Christianity under Roman rule. The Anglo-Saxon invasions separated the British church from European Christianity for centuries, so the church in Rome had no presence or authority in Britain, and in fact, Rome knew so little about the British church that it was unaware of any schism in customs. However, Æthelberht would have known something about the Roman church from his Frankish wife, Bertha, who had brought a bishop, Liudhard, with her across the Channel, and for whom Æthelberht built a chapel, St Martin's.
In 596, Pope Gregory the Great sent Augustine, prior of the monastery of St. Andrew in Rome, to England as a missionary, and in 597, a group of nearly forty monks, led by Augustine, landed on the Isle of Thanet in Kent. According to Bede, Æthelberht was sufficiently distrustful of the newcomers to insist on meeting them under the open sky, to prevent them from performing sorcery. The monks impressed Æthelberht, but he was not converted immediately. He agreed to allow the mission to settle in Canterbury and permitted them to preach.
It is not known when Æthelberht became a Christian. It is possible, despite Bede's account, that he already was a Christian before Augustine's mission arrived. It is likely that Liudhard and Bertha pressed Æthelberht to consider becoming a Christian before the arrival of the mission, and it is also likely that a condition of Æthelberht's marriage to Bertha was that Æthelberht would consider conversion. Conversion via the influence of the Frankish court would have been seen as an explicit recognition of Frankish overlordship, however, so it is possible that Æthelberht's delay of his conversion until it could be accomplished via Roman influence might have been an assertion of independence from Frankish control. It also has been argued that Augustine's hesitation—he turned back to Rome, asking to be released from the mission—is an indication that Æthelberht was a pagan at the time Augustine was sent.
At the latest, Æthelberht must have converted before 601, since that year Gregory wrote to him as a Christian king. An old tradition records that Æthelberht converted on 1 June, in the summer of the year that Augustine arrived. Through Æthelberht's influence Sæberht, king of Essex, also was converted, but there were limits to the effectiveness of the mission. The entire Kentish court did not convert: Eadbald, Æthelberht's son and heir, was a pagan at his accession. Rædwald, king of East Anglia, was only partly converted (apparently while at Æthelberht's court) and retained a pagan shrine next to the new Christian altar. Augustine also was unsuccessful in gaining the allegiance of the British clergy.
Æthelberht of Kent