The quotation that Beowulf exclaims, "fate will unwind as it must!" comes from early in the text as what follows is a discussion of Beowulf's father: "Your father's countrymen were afraid of war, / If he returned to his home,...." In this quotation, Beowulf is contemplating death at the hand of Grendel: "Now Grendel and I are called / Together, and I've come." Consequently we infer that Beowulf speaks of "fate" in the sense of the inescapable death he may face as the fate that is in store for him.
Fate generally refers to one of two concepts. It refers to events that occur in life in a manner that is beyond control: e.g., the encounter happened because fate ordained it. This view of fate, as with the other view, has a religious undertone as it originates in pagan religions. Fate also refers specifically to the time and manner of a person's death: e.g., Beowulf met his fate at the dragon's claws.
What we can infer from Beowulf's words regarding his potentially fated death is that he believes that a brave and honorable life will lead to a death with honor and without fear. We can infer that paganism has formed his belief system because the Christian doctrine regarding predetermined death is that it is in the hands of God, who is figuratively called "Providence." Thus if Beowulf had said "Providence will direct as it must!" we could easily infer he believed in Christian teachings.
From this limited statement, "fate will unwind as it must!" we can infer only that he believes in honor in battle leading to honor in death and that he was taught and believes in the pagan idea of inescapably fated death. We also infer as a corollary of the first two that Beowulf has no fear of nor regrets associated with a fated death or with the honor of battle or with death in battle.
Considering that Beowulf, according to much scholarship on the topic, was originated in the 700s (the eighth century) and that Christian teaching began to spread in Denmark in the 800s, being officialized by King Harald Bluetooth in 950, it would be surprising if Beowulf were thinking of and referring to a Christian concept of predetermination in death. The question of how much of Beowulf is Christian and, conversely, how much is pagan is one of the intriguing areas of debate among scholars. It is widely believed that those who transcribed Beowulf (the text as we have it today) either altered it or transcribed a version that had been altered over the years to contain Christian themes and elements. Thus the portrait of Beowulf's character in the text is sometimes contradictory; while some things he says seem to indicate a belief in Christianity, other lines hark back to pagan origins, like the one in question here ("fate will unwind as it must").
By about 700 AD, Willibrord, the ‘apostle of the Netherlands’ was already carrying out missionary work among the Danes, but it was only with the missionary activities of Ansgar, from 826, that Christianity gained a foothold in Denmark. (Danishnet)
The Anglo-Saxons, the culture depicted in Beowulf, believed in fate (or wyrd). With this fate was a belief in God. Therefore, Beowulf believed that God would decide what would happen to him in every battle. He entered in each battle with the knowledge that the God would "decree success on whichever side seems right to Him!” This mirrors the quote in question ("fate will unwind as it must").
One can infer that Beowulf was a true believer in God and fate itself. Everything that he did, he did for the glory of God. Therefore, he did not question any outcome or any battle. He only entered into the battle knowing that God would choose the victor. With this, Beowulf possessed no fear of death. He believed that if he died it was what God wanted. Beowulf gave all of his trust to God and the power God bestowed upon him.