A good question and a good answer from comment section on blog post:
First time commenter, long time reader. Great post!
How would you use the Presuppositional approach with world religions that hold similar (not identical) distinctions to Christianity? For example, RCC, Islam, or Mormonism.
Mike: This is an excellent question, Alex.
The presuppositions of the Christian are based on the existence of God and the authority of Scripture. Because these groups (and moreso RCC and Mormonism than Islam) all at least pay lip-service to both of those presuppositions, a lot of times a presuppositional practice just looks like appealing to the authority of Scripture.
Also, there’s another angle to try with Islam and Mormonism (and JWs as well), since they deny Trinitarian Monotheism. Denying that God exists as One Being eternally in Three Persons, and thus in relationship with Himself—precludes the
possibility of a God who is consistently both absolute and personal. Indeed, an absolute, personal God must by definition be absolutely personal.
Yet “absolutely personal” is almost a logical contradiction in terms. For a being to be absolute, he must be entirely unaffected, unable to be acted upon and never responding to another being. And for a being to be personal, he must interact in relationship with other beings, being affected and responding to the actions and affections of those with whom he shares a relationship.
How can any being can be absolutely personal, then? He must exist as a Trinity—an absolute Being entirely transcendent over His creation, yet existing in three co-eternal, co-equal Persons who interact with each other and enjoy each other in their eternal relationship (cf. John 17:5). Though they might claim otherwise, the Unitarian god of Islam and the Watchtower Society, and the polytheism of Mormonism, cannot consistently be both (a) absolute and (b) relate to humanity in a personal manner, because there is no such relationship within his own absolute essence, no such unity existing in diversity. Unitarianism is unity only, and polytheism is diversity only. Only Trinitarian Monotheism—only the God of the Bible—can account for absolute moral standards, which require an absolutely personal moral lawgiver, and for unity amid diversity in the creation (e.g., various species of dogs, birds, trees, etc.). In other words, reality logically presupposes the Trinitarian God of the Bible.