What are some of the philosophical issues in Christianity?

First there would be the philosophical issue of theism in general: Namely, the existence of God. Philosophical reasoning for theism has historically been given in the form of cosmological, teleological, axiological, and/or ontological arguments. Generally these arguments are given together as pieces of a whole foundation of reasoning. Second there is the issue of the "coherence of theism" or the analysis of the concept of God. This deals with not only the characteristics or attributes which define the "nature" of the concept of God, but also with how they are all rationally interconnected. Generally the attributes are as follows: Necessity, aseity, incorporeality, omnipresence, eternity, omniscience, simplicity, immutability, omnipotence, and goodness. These attributes, far from, arbitrarily selected, usually follow necessarily from either the nature of the arguments put forth for God's existence, or from one another.

Moving more specifically to Christian philosphy, there are topics of creation, providence, and miracle. There are philosophical issues involving the Trinitarian God: Logos Christianity, Modalism, Arianism, Anti Social Trinitarianism, Social Trinitarianism, Functional Monotheism, Group mind Monotheism, etc. Also concerning the incarnation and Christology. Furthermore, Christian theology has it's own subcategories of thought and study such as Theology Proper, Pneumatology, Revelation, Creation, Ecclesiology, Soteriology, Eschatology, Prayer and meditation, Missiology, Patriology, Doxology etc. all of which regularly involve philosophical study. The thought and beliefs which characterize a Christian worldview obviously have implications as well for numerous areas within areas of philosophy which are not of themselves necessarily Christian subjects: the subject of morality, for instance, or metaphysics and ontology would be important ones.

Christianity is a unique realm of thought, experience, and praxis and as such it can, and regularly does employ philosophical principles within it's own reflection. However there are no specifically "Christian" principles of philosophy.