So far I have read "Either/or", "Repetition", Plato's dialogues, "Nicomachean Ethics", "Meditations" by Marcus Aurelius and I'm quite familiar with Christianity. Which other primary texts or some kind of secondary source do I need to be familiar with in order to understand this text?
Me again. For this one, I don't think you need to read anything! You could do that whole process I described for reading the Sickness, then read the Sickness, and I think you'll be in the best possible position to read the Attack, but I don't think you need any secondary literature to help you read this one. I would however suggest at least a bit of familiarization with Kierkegaard's concepts, terms, and life story. These can be found on Youtube; I think the best one is by Eric Dodson.
Kierkegaard in 19 Minutes (Youtube video essay).