Beowulf is an epic tale of bravery and adventure. It also reflects the Anglo-Saxon culture's values in terms of its heroes: self-sacrificing, honorable, valiant and honest.
Perhaps the poem's unity comes from the way it was initially passed along: through the oral tradition. In other words, many tales were passed on verbally by sailors, travelers and scops (storytellers) before they were written down. While this process may have changed some of the details, the story remained (we believe) fairly intact until it was finally written down. Repetition provided the poem with unity. The earliest remaining copy is in England—
Beowulf survives in one manuscript, which is known as British Library, Cotton Vitellius A.15.
In terms of its origins, scholars consider that it may have originated any time between the end of the 7th Century and the beginning of the 11th Century, when the only existing manuscript was written. Another element that would not have been included were references to God and religion. These would have become a part of the story after the Roman Catholic Church sent its emissaries to make believers of the various barbaric and heathen races that inhabited the British Isles.
Part of the poem's complexity comes from how it was first perceived by scholars: not as a piece of literature—
...for over 100 years study focused on Beowulf not as poetry, but on what it revealed about the early Germanic tribes and language (philology).
For this reason, it has had an added depth for many years, becoming popular as literature only in the 1930s, very much influenced by J.R. Tolkien (The Lord of the Rings, et al), who was fascinated by the piece. Of the approximately 30,000 lines of poetry from the Anglo-Saxon period (449-1066), a little more than nine percent is represented by Beowulf (which has 3,182 lines).
Another piece of Anglo-Saxon literature, called "The Dream of the Rood" (a "dream-vision" poem in English), gives us insight into Beowulf.
By portraying Christ as a warrior-king, “The Dream of the Rood” represents a common trend in Anglo-Saxon literature: using the heroic diction of Old English to help make Christianity more acceptable to the Anglo-Saxons.
This not only accounts for Christian themes included in Beowulf, but also demonstrates how the language reflects the Anglo-Saxon cultural values: great warriors and strong leaders.
In terms of the story's universality, consider its themes: fortitude and wisdom; glory and treasure; fate and providence; loyalty, vengeance and feud; and, evil and monsters. Values of today, in terms of heroism (as seen in literature, film and life), still praise fortitude and loyalty. An element faced through enemies (regarding war, and in literature/film) includes evil, feuding and sometimes—in some form—a "monster". Themes that draw the interest of countless people include the attainment of glory and treasure; a code that hopes for vengeance against wrongdoers; and, the struggle to understand the concept of fate. In the United States, the public expects bravery and honor from its President. After 9-11, the American public thrived on the hope of vengeance. Osama bin Laden and many others, have been perceived as monsters.
Universal themes included protecting women, children and the weak. Victory in a fair fight was admired, but not bragging; murdering others for personal advancement was rejected on moral grounds. Dying to save others was lauded. These are also modern ideals.