One of the things that makes Heidegger's Being and Time so difficult for many modern readers is that the question of what it means to be seems like such an odd question to ask. What does it even mean to ask such a question? Does the question even mean anything at all, or is it pointless and vacuous philosophical navel-gazing? One of the difficulties in understanding Heidegger's thought is the tendency of modern man to think in terms of an object. As Michael Gelven points out, when we ask with Heidegger what it means to be, we are not asking what kind of an object something is; rather, we are asking what it means to be in the first place.
Gelven points out that one of the keys in grasping this difference in Heidegger's work is understanding the difference between the infinitive form of the German verb "to be," Sein, vs. the gerund form, seiend/Das Seiende. He notes that Heidegger's translators Macquarrie and Robinson translate Sein as "Being" and Das Seiende as "existent." The key to grasping this distinction is understanding that "Being" is a verb in the former place, whereas it functions as a substantive in the latter (Gelven, 1970).
Heidegger systematically articulates the distinction being Sein and Das Seiende through numerous trajectories. Michael Gelven expertly lays out this distinction on p. 19 of his commentary: Types of inquiry which focus on the question of what it means to be are called ontological inquiries, whereas those which focus on specific entities are called ontic. Terms of inquiry which focus on the question of the meaning of being are called existentials, whereas those which focus on specific entities are called categories. The status of the occurrence of an inquiry with respect to what it means to be is called a factical inquiry, whereas it is known as a 'factual' inquiry when it is focused on a specific object. The type of self-awareness involved in such an inquiry is called 'existential' when it has to do with the meaning of what it means to be, whereas it is called existentiell when it refers to a specific entity (Gelven, 1970).
The point Heidegger intends to make is that we are mistaken if we, like the logical positivists, think that the only legitimate way of investigating reality is by investigation of distinct objects or beings. Our investigations cannot be purely factual if they are to get at the heart of the question of what it means to be. They must instead entail factical investigation as well. Since "Being" is not an object, it is not a thing that can be subsumed under a broader category. Indeed, one might say that in Heidegger, our answer to the question of what it means to be determines how we describe and interpret all subsequent individual objects of our experience.
But what does this mean for the Christian presuppositionalist? The answer ought to be obvious. As the Christian presuppositionalist knows well, we all have our own distinct worldviews and pre-interpretive grids. In Heideggerian language, we might say that according to a purely 'factual' analysis of a bird, the Christian and the Buddhist might give strikingly similar descriptions of the physiological anatomy of a bird. But from a 'factical' perspective, that is, when it comes to interpreting the existential significance of the bird in terms of the question of what it means to be, we would give strikingly different descriptions in light of our different worldviews. The Christian would describe the bird as having been designed teleologically by God for his purpose, fallen under sin and therefore subject to inevitable decay and dissolution, and so on. The Buddhist, however, might think of the bird in terms of non-self, lacking a category for 'sin', not viewing its inevitable decay in terms of such punitive guilt, and so on.
One of the most commonly cited words in Heidegger's lexicon is Dasein: Gelven explains the word's meaning and significance:
"Why Heidegger chooses the human existent as the object to be investigated requires a consideration of that all-important term Dasein. Macquarrie and Robinson have wisely left it untranslated. Basically, in common, everyday German discourse, the term refers to human existence. This meaning is not completely abandoned in Heidegger; but he emphasizes its etymology as well. The term consists of two parts - da, meaning "here," and sein, "to be." Thus the etymologically derived translation of the term is "to be here." Some translators have chosen to use such phrases as "to be there," noting that at times the German term da can be translated by the English "there." Da, however, must be distinguished from dort, which means "there" in the sense of "at that place - not here." Da basically means "here," as in the simple German sentence Da kommt er: "Here he comes." The English language would allow such a sentence to run: "There he comes." Nevertheless, I suggest that to e mphasize the "there" over the "here" is distortive.
In light of this etymology, I think it is likewise incorrect to translate Dasein as "human being," because "human being" refers to an actual entity or existent, with reference to its genus and species. To inquire about a "human being" would be to engage in an ontical inquiry. What Heidegger means by the term Dasein is that entity which is capable of inquiring into its own Being, and indeed, such an inquiry into its Being is what makes Dasein what it is. Much of the examintion of Being and Time is directed toward a "definition" of Dasein, so I shall not try to give a capsule definition at this time.
The whole point about Dasein is that it itself can wonder about itself as existing. As will be seen later on, the meaning of existence can be significant only to one who asks about his own existence. For this reason, the question of Being itself is possible only because Dasein can reflect upon its existence. The mode of scientific (ontic knowledge cannot in principle examine such a question"(Gelven, 1970, bold mine)
The bolded part of the text is the key. The essence of what it means for the human being to be Dasein, for Heidegger, is that it can ask itself what it means to be. In the words of Gelven, "it is meaningful...to inquire about the meaning of Being, since there is another type of inquiry - the ontological. In this latter way, Dasein reflects upon itself as existing"(Gelven, 2007).
Gelven, Michael. A Commentary on Heidegger's Being and Time: A Section-by-section Interpretation. New York: Harper & Row, 1970. 18-19, 23, 24. Print.