In my experience the presuppositionalist program of setting out to defend specifically Christian theism generally produces scoffing rather than interaction. At the beginning of his debate with Gordon Stein, Greg Bahnsen states his position on this matter. He says, “I want to specify that I’m arguing particularly in favor of Christian theism, and for it as a unit or system of thought and not for anything like theism in general, and there are reasons for that.”
There are at least two senses in which Bahnsen explains he will not be arguing for or defending theism in general. Bahnsen will argue for Christian theism as a unit or system of thought. This is a distinctively presuppositional approach to the “question” of God’s existence over against other approaches which take the existence of God to be a fact not unlike other facts especially in the sense that it may be proven apart from other worldview considerations. Secondly, Bahnsen will argue for and defend Christian theism as opposed to theism in general. This is the clearer sense of what Bahsen is saying and is the topic of this post. Bahnsen goes on to present three reasons for this approach.
The first reason Bahnsen gives is that the “various conceptions of deity found in world religions are in most cases logically incompatible, leaving no unambiguous sense to general theism – whatever that might be.” This is to say that, for example, the conception of Allah is logically incompatible with the conception of the Christian God. Allah does not exist as three persons while the Christian God does. Various conceptions of deities in world religions claim to be the only or the true. There are of course many more examples here. There are almost as many examples as there are world religions. The term “general theism” is a bit like “non-denominational” in the sense that most non-denominational churches have created their own denomination through what they believe and their informal or perhaps formal associations with other churches. (If the analogy is unhelpful or offensive throw it out.) The god of general theism is just one more conception of a non-Christian deity which is ambiguously defined usually in an ad hoc manner at the end of some direct classical proof.
Secondly Bahnsen says, “I have not found the non-Christian religions to be philosophically defensible, each of them being internally incoherent or undermining human reason and experience.” This is a pretty clear statement that leaves little room for comment. General theism, whatever it is, would presumably be included in or composed of this category given what Bahnsen has already said. Bahnsen does not believe that he can defend any of these non-Christian religions and his opponent in this context, atheist Gordon Stein, has little reason to challenge Bahnsen’s findings. One is immediately left wondering why Bahnsen would want or need to defend any of these non-Christian religions anyway, which leads to his third stated reason for approaching this particular area of the debate the way that he does.
Bahnsen explains, “Since I am by the grace of God a Christian, I cannot, from the heart, adequately defend those religious faiths with which I disagree.” He continues, “My commitment is to the Triune God and the Christian world view based on God’s revelation in the Old and New Testaments.” Here Bahnsen presents an ethical reason for abstaining from the defense of non-Christian religions and conceptions of God. To do so is to fail in his commitment to God. It is not insignificant that Bahnsen views his faith as being a result of the grace of God. It is likewise not insignificant that he mentions the Triune God, the Christian worldview, and God’s revelation in the Old and New Testaments. He explicitly states his presuppositions and makes the clear statement, “I am defending Christian theism.”
Disagreeing with Bahnsen’s approach as described here is therefore no small matter if Bahnsen’s reasons for using this approach are not answered satisfactorily. Further, if Bahnsen is correct and what he says is based upon Scripture as he has argued in many other places, then one risks undercutting everything he or she is attempting to argue for or defend in terms of a Christian apologetic.
You may find the debate transcript I am quoting from here: http://www.bellevuechristian.org/faculty/dribera/htdocs/PDFs/Apol_Bahnsen_Stein_Debate_Transcript.pdf