1. Natural theology vs. revealed theology

The boundary between the two is contested. Aquinas held that reason can establish the existence of God and some of the divine attributes, but cannot establish the Trinity or the Incarnation without revelation. Maimonides drew a similar line in a Jewish frame, reserving to reason what reason could secure and to prophetic revelation what exceeds it. Al-Farabi and Ibn Sina worked out similar distinctions in the Islamic tradition. Reformation theology, especially in its Barthian twentieth-century version, rejected natural theology entirely; Karl Barth's "Nein!" to Emil Brunner in 1934 is the famous modern rejection, on the ground that natural theology underwrites a God built to human specifications rather than the God who speaks from outside.

2. Classical arguments for the existence of God

Cosmological arguments

Cosmological arguments argue from the existence or order of the cosmos to a cause that explains it. Two major families.

Kalam cosmological argument. Developed by medieval Muslim theologians (especially al-Ghazali), revived in modern analytic philosophy of religion by William Lane Craig. The form:

1. Whatever begins to exist has a cause.
2. The universe began to exist.
3. Therefore the universe has a cause.

Premise (2) is argued philosophically (the impossibility of an actual infinite regress) and empirically (appealing to Big Bang cosmology). Premise (1) is argued as a metaphysical principle. The argument is disputed at both premises.

Aquinas's Five Ways. Aquinas offers five short arguments in Summa Theologiae I.2.3. The first three are cosmological: from motion (there must be a first unmoved mover), from efficient causation (there must be a first cause), from contingency and necessity (there must be a necessary being). The fourth is from degrees of perfection; the fifth is a teleological argument from the ordering of natural processes. These arguments rest on Aristotelian metaphysical premises that no longer have the default status they had in the thirteenth century.

Ontological arguments

Ontological arguments argue from the concept of God to God's existence. Anselm's version in the Proslogion (1078):

1. God is that than which nothing greater can be conceived.
2. A being that exists in reality is greater than a being that exists only in the mind.
3. Therefore God, than which nothing greater can be conceived, must exist in reality.

The argument has been criticized since Gaunilo (contemporary of Anselm), reformulated by Descartes, criticized again by Kant (existence is not a predicate), and reformulated again in modal logic by Plantinga in the twentieth century. Plantinga's version is valid but its key premise (that there is a possible world in which a maximally great being exists) is disputed.

Teleological arguments

Teleological arguments argue from apparent design in nature to a designer. The classical form is Paley's watchmaker (1802). The modern form, after Darwin, typically avoids biological examples and argues from the fine-tuning of physical constants — the values of the gravitational constant, the cosmological constant, and others are such that small changes would make a life-bearing universe impossible. The inference to a designer is contested: the leading alternative is the multiverse hypothesis, on which we happen to be in a life-bearing region of a much larger physical reality.

Moral arguments

Moral arguments argue from the existence of objective moral obligation to a ground of moral obligation beyond human conventions. Kant's version is that the moral law presupposes the possibility of a highest good, which requires God and immortality as postulates of practical reason. C. S. Lewis's popular version argues from the universal experience of ought-claims to a transcendent lawgiver. The argument is contested by those who hold that moral facts can be grounded without theological premises (constructivism, natural-law-without-theism, etc.).

3. The problem of evil

The single most weighty argument against theistic belief. Classical formulation (attributed to Epicurus through Hume):

1. If God is all-good, God wants to prevent evil.
2. If God is all-powerful, God can prevent evil.
3. Evil exists.
4. Therefore there is no all-good, all-powerful God.

The argument has two forms. The logical problem claims the existence of any evil is incompatible with theism. This form is usually considered defeated by Plantinga's free-will defense: it is at least possible that an all-good, all-powerful God would permit some evil for the sake of genuinely free creatures. The evidential problem is harder: even if the logical problem fails, the sheer quantity and distribution of suffering — especially the suffering of innocents and non-human animals — is claimed as strong evidence against theism. This form remains a live debate.

Theodicies (defenses that explain why God permits evil) fall into families: free-will theodicies, soul-making theodicies (John Hick), privation theodicies (Augustine), skeptical theism (we should not expect to understand God's reasons). Each has its weaknesses. The Jewish tradition after the Holocaust has produced distinctive theodicy-refusing responses — Emil Fackenheim, Richard Rubenstein — which hold that some forms of evil resist any theological explanation.

4. Divine attributes

Philosophical theology analyzes the classical attributes of God one by one and asks what they could coherently mean.

These attributes are not independent; a change to one forces changes elsewhere. Process theology, for example, gives up simplicity, impassibility, and exhaustive foreknowledge together in exchange for a God more directly involved in temporal becoming.

5. Jewish and Islamic philosophical theology

Medieval Jewish and Islamic philosophical theology is not a footnote to Christian scholasticism — it is an independent tradition that Christian scholasticism drew on. Maimonides in the Guide of the Perplexed (c. 1190) works out a rigorous doctrine of divine attributes using the negative method: we can say what God is not, not what God is. Ibn Sina (Avicenna, d. 1037) develops an argument from contingency that directly influenced Aquinas. Al-Ghazali (d. 1111) attacks the philosophers in Incoherence of the Philosophers, defending a theological use of reason that is nevertheless skeptical of philosophical overreach. Ibn Rushd (Averroes, d. 1198) replies to al-Ghazali in Incoherence of the Incoherence. The whole conversation is as sophisticated as any in the history of philosophy.

For a contemporary student, one important effect of taking this history seriously is that the "analytic philosophy of religion" debates of the twentieth century look less novel. Many of the moves were already on the table a thousand years ago, in more than one language and religious frame.

6. Religious epistemology

A more recent question: under what conditions is religious belief justified? Two major positions.

Reformed epistemology (Plantinga, Wolterstorff) holds that belief in God can be properly basic — held without argument, yet justified — much as belief in other minds or the past is properly basic. If there is a cognitive faculty that produces religious belief in response to appropriate circumstances (a sensus divinitatis), then religious belief formed through that faculty is justified absent defeaters.

Evidentialism (Clifford's "it is wrong, always, everywhere, and for anyone, to believe anything upon insufficient evidence"; in religion, think of the "New Atheist" position) holds that religious belief is justified only if supported by sufficient evidence, and typical religious belief falls short.

The debate turns on general questions in epistemology — what is evidence, what is justification — and has no consensus resolution. Notable recent contributions include Richard Swinburne's probabilistic natural theology and John Hick's religious-pluralist epistemology.

7. Common mistakes

8. Cross-references

• aquinas agent: The Five Ways, analogy, the relation of philosophy and theology.
• maimonides agent: Negative theology, parables, the Guide of the Perplexed.
• augustine agent: The Augustinian account of evil as privation.
• huston-smith agent: Comparative perspectives on divine attributes across traditions.
• systematic-theology skill: Doctrinal architecture that philosophical theology clarifies.
• scripture-and-interpretation skill: The exegetical base for revealed-theology premises.

9. References

• Plantinga, A. (1974). The Nature of Necessity. Oxford University Press.
• Swinburne, R. (2004). The Existence of God, 2nd ed. Oxford University Press.
• Hick, J. (2010). Evil and the God of Love, reissue. Palgrave Macmillan.
• Maimonides, M. (c. 1190). The Guide of the Perplexed. Trans. Pines.
• Al-Ghazali (1095). The Incoherence of the Philosophers.
• Ibn Rushd (c. 1180). The Incoherence of the Incoherence.
• Rea, M. and Murray, M. (eds.) (2008). Analytic Theology. Oxford University Press.
• Rowe, W. L. (1979). "The Problem of Evil and Some Varieties of Atheism." American Philosophical Quarterly 16.