Many excellent points have already been made. To attempt one more, there is a distinction between knowing that Christianity is true and defending that Christianity is true.
1. Knowing that Christianity is true does not require argumentation. In fact, there are various definitions of "knowledge." Depending on context, more than one of these might even apply to one's belief that Christianity is true; I might "know" that Christianity is true in more than one sense (e.g. internalism vs. externalism, infallibilism vs. fallibilism).
2. Defending that Christianity is true will often entail argumentation (the realm of what we typically consider to be apologetics)... although one might also say that simple, unspoken obedience to God - even to the point of martyrdom - is an indirect witness to and defense of one's public faith that does not involve syllogistic reasoning.
When I was young, I conflated 1 and 2. While I did not waver in my faith - I've always considered Christianity better than any alternative I have encountered or considered - I did not have full, "epistemic" (as opposed to a mere, psychological "feeling") assurance that Christianity was true. In my mind, there were always more alternatives to Christianity than I could concretely, specifically, internally critique. In my mind, there was no transcendental argument which uniquely selected for Christianity. Underlying my sense of unease was that I needed to argue my way to having full, "epistemic" assurance that Christianity is true. On the contrary, I now think that I defend knowledge I already have had.
Thus, there might be called varieties of presuppositionalism:
A) "Epistemic" presuppositionalism, in which one recognizes that he makes knowledge claims on the basis of assumptions or presuppositions which may be intrinsically "justified" (the full meaning and applicability of which will depend on context), "justified" without the need for argumentation. In this way, one may have "knowledge" (even in the sense of "full, epistemic assurance") that Christianity is true without having to critique infinitely many worldviews or rely on inferential reasoning.
B) "Apologetic" presuppositionalism, in which one defends his presuppositions by intentionally provoking reflection upon his or other's presuppositions: e.g. attempting to show how his presuppositions are coherent and answer certain questions, how other's presuppositions are incoherent, unable to answer certain questions, or incompatible with what else they may claim to believe, etc.
Interestingly, the more one articulates A) in the sense of fleshing out one's exact meaning, the more one will tend to actually be performing B). For example, in a fuller discussion of what it means for something to be intrinsically "justified," one might differentiate and expound on foundationalism in contrast to coherentism, infinitism, and positism. In so doing, one might defend the former (and, by proxy, his alleged intrinsically justified "knowledge") against the latter. One's apologetic will always have a tendency of attempting to confirm what one already "knows." I find this interesting because it shows how circularity might appear in one's apologetic but not in one's structure of knowledge. This might bridge a gap between Clarkian and Van Tilian species of "presuppositionalism."
With all that said, an advantage of one's affirming presuppositionalism of variety A) is that presuppositionalism of variety B) need not be the only apologetic of which one might avail himself. For example, one's internally and epistemically "justified" presupposition might legitimatize other modes of "knowing" (e.g. empirical) and, therefore, other modes of apologetic defenses (e.g. "evidentialism").
