Kierkegaard appears to be as Chrisitian and religious as any philosopher? Nevertheless, there are at least two senses in which he was an a-theist.
1. He was no mere Theist. A Theist refers to a God who intervenes in the universe with omnipotent power. For Kierkegaard, that belief in itself was mere paganism. Religion must be Faith, in which the individual is transformed.
2. In light of 1, the claim that Kierkegaard was a-theistic may seem like a mere play with words, which is just a superficially paradoxical way of saying that Kierkegaard was a Fideist, that is he had a theology of faith. But, the rigour with which Kierkegaard pursued 1. leads towards atheism in the normal sense.
What theological commentators like to call Kierkegaard's 'fideism', or possibly his 'divine commandment ethics', is Kierkegaard's definition of faith as subjective experience and as experience of subjectivity. Some would like to relegate such views to Kierkegaard's 'aesthetic' or 'pseudonymous' works such as Fear and Trembling and Either/Or, which are considered as merely the way to Kierkegaard's religious point of view. One response to this, is that there is only the way and there is never more than the way in Kierkegaard, but today I will concentrate on something else.
I spent a lot of time last semester considering Works of Love in an ethics class. I structured an ethics class to contain consideration of virtue and reason in Plato, friendship in Aristotle, sympathy in Hume, universality in Kant, the Utilitarian Maxim in Hegel, the superiority of ethical life to subjective morality in Hegel, all the ethical issues in Nietzsche's On the Genealogy of Morality including the relation between master and slave.
In this context, it is particularly clear that Works of Love is a text concerned with defining ethics. The answer seems to be straightforward: Ethics is based on Christian love, inparticular the commands 'You shall love' and 'Love your neighbour as yourself'.
But what comes out of Kierkegaard's exhaustive analysis of these commands? He is finding a response to Kant's suggestion that ethics derives from the possibility of a universal command. For Kierkegaard, Kant's kind of command lacks a place for the individual who commands and is commanded. What does a universal command mean to me, or to you, or anyone?
The command to love makes it clear where the individual fits in. The individual must love the self in loving the neighbour. I can only love my neighbour if I love myself. Self-love is already a relational love. I become the subject and object of love. The command to love myself is not an abstract irrelevance since only in the self-relation of self-love can there be an individual. Individuality which is more than the moment to moment of some pure flow of perceptions in experience must be the relation of empirical self with something in itself more than the moment, but which can be contained in the moment.
What is there in Kierkegaard's discussion of love which takes us beyond subjectivity? All love with the individual outside myself is intertwined with my own self-relation. There can only be love where there is love of self. That applies to God as well. Love of God is a relation with the absolute, and my relation with the absolute must be the relation with the self which exists as more than the moment.
The theological reader of Kierkegaard may assert that Kierkegaard refers to love of, and obedience to a God outside subjectivity, but can the theologian show this in the detail of Kierkegaard's philosophy, other than those moments when Kierkegaard uses the formulae of Christian theology. His thought is never in the formulae, it is always in the conditions of subjectivity, of an individual who experiences individuality.