Justus (died on 10 November between 627 and 631) was the fourth archbishop of Canterbury. Pope Gregory the Great sent Justus from Italy to England on a mission to Christianise the Anglo-Saxons from their native paganism; he probably arrived with the second group of missionaries dispatched in 601. Justus became the first bishop of Rochester in 604 and signed a letter to the Irish bishops urging the native Celtic church to adopt the Roman method of calculating the date of Easter. He attended a church council in Paris in 614.
Following the death of King Æthelberht of Kent in 616, Justus was forced to flee to Gaul but was reinstated in his diocese the following year. In 624, he was elevated to the Archbishopric of Canterbury, overseeing the dispatch of missionaries to Northumbria. After his death, he was revered as a saint and had a shrine in St Augustine's Abbey, Canterbury, to which his remains were translated in the 1090s.
Arrival in Britain
thumb|left|alt=An illuminated manuscript illustration of a central seated figure holding an open book. He is flanked by two colonnades filled with small scenes. Over the central figure is an arch which surmounts a winged bull.|The [[evangelist portrait of Luke, from the St Augustine Gospels (), which may have accompanied Justus to Britain]]
Justus was a member of the Gregorian mission sent to England by Pope Gregory I. Almost everything known about Justus and his career is derived from the early 8th-century Historia ecclesiastica gentis Anglorum of Bede. As Bede does not describe Justus's origins, nothing is known about him before he arrived in England. He probably arrived in England with the second group of missionaries, sent at the request of Augustine of Canterbury in 601. Some modern writers describe Justus as one of the original missionaries who arrived with Augustine in 597, but Bede believed that Justus came in the second group. The second group included Mellitus, who later became Bishop of London and Archbishop of Canterbury.
If Justus was a member of the second group of missionaries, then he arrived with a gift of books and "all things which were needed for worship and the ministry of the Church". A 15th-century Canterbury chronicler, Thomas of Elmham, claimed that there were some books brought to England by that second group still at Canterbury in his day, although he did not identify them. An investigation of extant Canterbury manuscripts shows that one possible survivor is the St Augustine Gospels, now in Corpus Christi College, Cambridge Manuscript (MS) 286. Another Gospel, in an Italian hand, and closely related to the Augustine Gospels, is MS Oxford Bodelian Auctarium D.2.14, which shows evidence of being held in Anglo-Saxon hands during the right time frame. Lastly, a fragment of a work by Gregory the Great, now held by the British Library as part of MS Cotton Titus C, may have arrived with the missionaries.
Bishop of Rochester
Augustine consecrated Justus as a bishop in 604 over a province including the Kentish town of Rochester. The historian Nicholas Brooks argues that the choice of Rochester was probably not because it had been a Roman-era bishopric, but rather because of its importance in the politics of the time. Although the town was small, with just one street, it was at the junction of Watling Street and the estuary of the Medway and was thus a fortified town. Because Justus was probably not a monk (Bede did not call him that), his cathedral clergy was very likely non-monastic too.
thumb|The beginning of the charter in Textus Roffensis
A charter purporting to be from King Æthelberht, dated 28 April 604, survives in the Textus Roffensis, as well as a copy based on the Textus in the 14th-century Liber Temporalium. Written mostly in Latin but using an Old English boundary clause, the charter records a land grant near Rochester to Justus's church. Among the witnesses is Laurence, Augustine's future successor, but not Augustine himself. The text turns to two different addressees. First, Æthelberht is made to admonish his son Eadbald, who had been established as a sub-ruler in the region of Rochester. The grant itself is addressed directly to Saint Andrew, the patron saint of the church, a usage parallelled by other charters in the same archive.
Wilhelm Levison, writing in 1946, was sceptical about the authenticity of this charter.
Æthelberht built Justus a cathedral church in Rochester; the foundations of a nave and chancel partly underneath the present-day Rochester Cathedral may date from that time. Although the letter has not survived, Bede quoted from parts of it.
In 614, Justus attended the Council of Paris, held by the Frankish king, Chlothar II. It is unclear why Justus and Peter, the abbot of Sts Peter and Paul in Canterbury,