The vikings' culture had not yet reached a point where it was a national thing. Their religion still relied on verbal passing down and local differences, as opposed to having centralized churches like the English kingdoms have.
When they meet the English, they see a religion that is far better organized than theirs. Which is foreign to them, and probably screams "cult" in their minds as they cannot comprehend how all the English they meet are so in tune with their religion (as opposed to having regional differing opinions).
To the vikings, that makes the English seem like mere mouthpieces, spouting the same religious nonsense (as they see it), and turns their perception of the English to mindless religious zombies as opposed to individuals with their own opinions of their religion.
They're also not used to a monotheistic religion, and if I recall correctly openly mock having a single god in a particular scene, with Floki (I think) saying something like "how can he be the god of everything?"
And then there's just the general dislike of them because they are an enemy ripe for raiding. Most men other than Ragnar have raiding on their minds, so they demonize their enemy. What better way to demonize them than by calling on religious superstition?
Floki seems to be the instigator for these types of events, often using Norse religion to band together the Vikings, and adding a hate for the English/Catholic at the same time. He never forgave Rollo for getting baptized (even though Rollo mocked the baptism and never took it seriously), and never got over his dislike for Athelstan.
He often starts the fighting. Think back on the first meet between the vikings (just landed) and the English. The English guard takes off his medallion and gives it to Ragnar/Rollo (I forget?) as a sign of peace and trust (which is a big thing, given you have armed soldiers landing on your beach). Floki then lunges forward and forcibly takes another trinket for his own. That changes what was a friendly encounter into one where the Vikings are seen as greedy.