The 'problem' of proving the existence of God, in the case of the divinity of Christ can actually be explored. From a purely philosophical notion of thought, the existence of Jesus himself reflects on the "economic" formulation of the doctrine of the Holy Trinity, as God has chosen to communicate himself with humanity, and the reason why He does this is the "immanent" formulation of the doctrine of the Holy Trinity. Thus, it is strictly speaking possible to link the idea of a Kantian thought about the "transcendental" experience, as proof alone of the existence of God. This is because human beings are finite in their knowledge, and so the transcendental experience, is unexplainable. This is how we prove God's existence using the "absolute mystery" argument and line of thought. The proof of Jesus as the Son of God is in that alone. The fact that God has revealed Himself to humanity - creating a link between incarnation and grace. Grace in itself is proof of the existence of God, simply because the revelation of God's self to human beings is incarnate in the word, providing an objective reality that is 'without' which elements our hearing, through the direction and guidance of the Holy Spirit, 'within'. Thus, grace is what constitutes human existence, and thereby stating the 'pure nature' argument, is pointless, as it will be "counter-factual." This is very very simple philosophy.