"The Seafarer," a poem that speaks to us down the centuries about suffering, loss, and hope for an everlasting life, is found in what is known as the Exeter Book and was most likely composed in the 8thC-9thC. As your question implies, the speaker, among other things, talks to us about his sense of loss for life as it was when he lived in a pagan, rather than Christian, world.
Many scholars view "The Seafarer," in part, as an elegy or at least elegiac--that is, a lament for something important that has been lost. Early in the poem, the speaker regrets the loss of his home and friends:
how I, wretched and full of sorrow, on the ice-cold sea/wandered in winter on the paths of an exile,/bereft of beloved kinsmen . . . (ll.14-16)
The seafarer speaks of his loss of home and family in powerful terms when he uses a word like the Old English bidroren, which is translated either as deprived of or bereft, words that imply a serious and heart-felt loss. Part of his lament for the past, then, is personal because he has lost his home and his friends. The truly elegiac tone, however, comes later in the poem when the speaker laments the loss of something even greater than a personal attachment.
When we try to understand Anglo-Saxon (Old English) literature, such as Beowulf, "The Seafarer," and "The Wanderer," we are constantly confronted by the tension between two belief systems--the paganism of pre-Christian Anglo-Saxons and Christianity. These two belief systems are not compatible with each other, but they appear next to each other in several Anglo-Saxon works as if they can be blended into one consistent belief system. But no matter how we interpret the poem--as an allegory of a spiritual quest for a Christian belief system or as a lament for the speaker's pagan past--the speaker himself clearly feels a great loss of his pagan past:
. . . The days are departed,/all the glories of the kingdom of the earth;/now there are neither kings nor emperors,/nor gold givers as there once were,/when they performed themselves so many glorious deeds. (ll. 80-85)
This lines speak to a pagan life that reflects the "glorious deeds" of warfare and kings who handed out gold to their loyal warriors. The seafarer's tone is of a man who is not completely in the moment, that is, he recalls a life and a belief system that is not consistent with the Christian beliefs he espouses at the end of the poem when he says
Let us consider where our true home is . . ./where life is long in love of God,/hope in the heavens. (ll.116-122)
Even though the speaker concludes the poem with conventional Christian piety, and the new belief system is clearly in the speaker's mind, one gets the sense that he regrets the loss of "glorious deeds" and "gold-givers" and is not completely convinced in the value of the world in which
. . . the weaker ones remain and rule the world,/gain the use of it by toil.