The problem is we can't actually date when King Arthur may have lived (or at least whoever he was based on) because the mythology is so complex and has changed alot since. He may have been a real historical figure such as the Arthur in the King Arthur (2004) movie or as Aurelius Ambrosius, or he may have been based on one of many British tribal warlords. The monks at Glastonbury Abbey claimed to have found the bodies of Arthur and his Queen in the late middle ages, but many historians suspect this was a hoax by the monks to draw pilgrims (the medieval version of tourists) to the region.
Some places in the legends have actually been found such as Baden Hill in Dorset UK where Arthur was said to have fought the Saxon chief Cedric, though Camelot's whereabouts are still conjecture.
The movie itself is set just after the Roman withdrawal from Great Britain and depicts a mixture of Roman Catholicism and Celtic Catholicism (which took Christian belief and mingled it with traditional beliefs) and a generic paganism (that is only conjecture based on descriptions by Roman historians such as Julius Caesar and Tacitus, good places to read but remember they were writing from the enemies point of view).
Other sources of Arthurian legend include Geoffrey of Monmouth's 12th century Historia Regum Britanniae (History of the Kings of England) and Malory's
'Le Morte d'Arthur' which was first composed in the mid 15th century. Thus the legends take on a Christian slant because that was what the audiences of the time believed and many traditional tales such as Arthur's have been adapted to incorporate Christian mythology, such as the inclusion of the Holy Grail . The Guenevere and Lancelot (and the Holy Grail) was introduced to the body of Arthurian legend by the 12th-century French writer Chrétien de Troyes.
There are older Welsh and Breton tales and poems relating the tales of Arthur, dated to earlier than Geoffrey and are usually termed "pre-Galfridian" texts. These depict Arthur as a more supernatural pagan figure who defended Britain from human and supernatural enemies alike or as a magical figure of folklore, sometimes associated with the Welsh Otherworld, Annwn. Tennyson later revived interest in the Arthur myth in early modern English literature.