We cannot be certain when the Anglo-Saxon poem Beowulf was composed, how it was created, or exactly when it was written down. What we can do is pay attention to top scholars in the field and make some pretty good guesses.
The only surviving manuscript of Beowulf is written in Old English (Anglo-Saxon). Rather than being composed at a specific time, the poem probably developed out of various influences, especially folk tales and traditions. Parts of it may have originally been performed by court poets or traveling bards (scops, pronounced "shops," in the Anglo-Saxon) who would have sung or chanted their poems to the accompaniment of a musical instrument such as a harp. We can conclude, then, that the work grew out of popular art forms, that various influences worked together, and that the story may have changed as it developed.
During the late 1920s and early 1930s, an American scholar named Milman Parry revolutionized the study of live performances of epics. He demonstrated convincingly that ancient Greek poems (the Iliad and the Odyssey) were composed in an oral-formulaic style based on tradition and designed to help the performer produce a long piece from memory or improvise material as he went along.
Francis P. Magoun, Jr., in his essay, "The Oral-Formulaic Character of Anglo-Saxon Narrative Poetry," published in the literary journal Speculum in 1953 (Vol. XXVIII, 446-467), demonstrates that the poems were recited or, more likely, sung or chanted, to audiences in the way that similar works are presented in Beowulf. An example in the epic itself is the performance of The Finnsburh Episode (lines 1063 ff.) when Hrothgar's scop honors Beowulf for his victory over Grendel. Magoun points out that the bards relied on language specifically developed for the poetry, formulas worked out over a long period of time and designed to fit the metrical demands of a given line while expressing whatever ideas the poet wished to communicate.
Although primarily a pagan poem, Beowulf contains Christian allusions that cannot be ignored. There is no mention of Jesus in Beowulf, and references to God seem based on the Old Testament rather than the New. But King Hrothgar and Beowulf sometimes refer to a single, all-powerful God, and there are instances of symbolic rebirth in the poem, such as Beowulf's emergence from the mere after his defeat of Grendel's mother. The fight with the dragon, late in the poem, especially seems to have Christian overtones. Counting the thief, Beowulf is accompanied by 12 associates, most of whom desert him (reminding us of Christ's apostles). We are told that God's will is done throughout the poem.
Still, many of the Christian references have the feel of afterthoughts. It seems more likely that they were added to the work as it developed — not necessarily by one scop or scribe but by several, all trying to make the poem more palatable to an increasingly Christian audience. The manuscript that we end up with is clearly influenced by Christian philosophy but remains heavily heroic.
What, then, can we conclude about the making of Beowulf? The poem was created in the oral-formulaic mode, based on folk tales and tradition, and probably composed as a whole sometime in the eighth century (700-800 ad) in England. The setting of the action in the epic is Denmark and Sweden in the fifth or sixth centuries, but the tone probably was altered to appeal to later audiences. Although a number of scops may have contributed to the poem's development, our version most likely was the creation of one poet. While the poem may have been altered over the 200 or more years before it was set down in writing, the oral-formulaic tradition would have limited the changes. It was written late in the tenth century (circa 1000 ad) by a scribe who probably was educated in a Christian monastery. He may have been a scop himself, or the work could have been dictated to him.
Beowulf may or may not be the first great heroic poem in English literature, as some scholars claim. It is, however, the one that survived.