Of all of the character arcs in Vikings, Athelstan's might be the most interesting. He goes from a free western monk, to a slave of Vikings, to free man who exists in his own perpetual state of no man's land. Athelstan's character explores the struggle of being torn between two belief systems, not only that of his religion but that of the cultures he has learned to call his own.
By the time of his death, Athelstan has at last found home in one of the religions, but whether not he would eventually tie himself down to a single community, viewers will never know. This is only one of many things viewers are left speculating about when it comes to Athelstan.
He Helps Ragnar While Still His Slave
Ragnar wouldn't have been as successful as he was without the help of Athelstan. Athelstan is kept as Ragnar's slave after being captured by him, as Ragnar wants to use him as a way of learning about the west. If Athelstan were wise, he would have declined to tell Ragnar about the geography of the west--and, the series suggests, Athelstan actually is supposed to be very smart--so why would he give away his people so easily?
He Pretends He Isn't A Christian
It doesn't make much sense that Athelstan pretends he isn't a Christian when the Vikings travel to make their sacrifices. If he does so because he is afraid of being found out for still holding his past faith, then this itself means his faith has dwindled as all other Christians in the show are seen to prove their belief through a willingness to die for it.
He Suddenly Believes In The Gods
Athelstan is meant to be at odds with two religions, and that's all well enough. It's strange how it happens, though. He seems very much devoted to his God, but between episodes, viewers find Athelstan suddenly describing how real the Viking Gods are to him, and how he has heard Thor's hammer in the sky, so he must be real--as if he never heard thunder before he went to Kattegat.
He Hits On Judith In Front Of The King
Judith tells Athelstan that she had a sex dream about him, to which he responds by comforting her that she didn't sin because she only imagined the actions.
He behaves mindful of the fact that she must not act on them, saying it's good when he leaves town so that she won't be tempted, but then later when he and Judith are bathing with Lagertha and King Ecbert Athelstan suddenly becomes heavily flirtatious with her, undermining everything viewers have known about his character.
He Raids With Ragnar
Athelstan may have grown used to living in Kattegat, but this still doesn't explain how he justifies going off to raid in the west and do the same to innocent people as was done to him when Ragnar murdered the unarmed monks around him and only kept him for the purpose of educating himself about the west.
He Sympathizes With Ecbert
Athelstan proves that he has the capacity to empathize with pretty much anybody, to a fault, it would seem. He falls under the wing of King Ecbert, who basically takes him on as a prisoner but simultaneously charms him with creative work, and this is all it takes for Athelstan to once again fall in love with his captor, as he did with Ragnar. The relationship is not so strong, but the basis is the same.
He Returns To Kattegat After Working For Ecbert
Working for King Ecbert brings Athelstan closer to his Christian faith again after years of being estranged from it. He even sympathizes with Ecbert at this point.
Its the perfect moment for Athelstan to reclaim his roots and recognize that he was possibly victim to Stockholm syndrome--sympathizing with captors in order to survive--when it came to his relation to Ragnar, but no. Athelstan is destined to love both captors, moving between Kattegat and the west at their beck and call.
He Sleeps With Judith
Athelstan knows very well the risk he is taking when he chooses to sleep with Judith. He is putting himself at risk if her husband finds out, but even more so he allows for the possibility that Judith herself could be killed, as women were punished more harshly. And on top of this, Judith could become impregnated and both she and her child could be murdered for her infidelity.
He Throws Away His Wrist Band
Athelstan finally chooses a faith--something that was surprising in the first place, as he seemed content to, at last, be at peace with embracing to seemingly opposing practices. Yet, one day light hurts his eyes and he decides it's a sign from God. The real shock is when he throws the wristband Ragnar gave to him as a symbol of accepting him as a free man in the culture. It seemed he was a man who could maintain being a Christian as well as a proud member of the Vikings.
He Isn't Suprised To See Floki
With his re-found devotion to Christianity Athelstan takes on a sort of divine contentment with the world. He puts up no fight when the community rejects him, and when Floki comes to his room to murder him he looks up at him with a look of passiveness, if not friendliness. It seems as if he almost expected Floki to come for him, which makes his death partly voluntary, and a willing departure from life with Ragnar.