The Christian message provides the answers to the questions implied in human existence. These answers are contained in the revelatory events on which Christianity is based and are taken by systematic theology from the sources, through the medium, under the norm. Their content cannot be derived from questions that would come from an analysis of human existence. They are 'spoken' to human existence from beyond it, in a sense. Otherwise, they would not be answers, for the question is human existence itself.*What is so cool to me is that this shows the necessity of a theistic God, that is, a God who is outside of and separate from man and his world, yet one who can definitively reveal himself and speak into man's world. If all we have is a deistic God (such as that of Thomas Jefferson or Isaac Newton), he may have the omniscience and wisdom to explain the human condition and the conundrums and pains of our experience, but he cannot speak to us or reveal to us his wisdom. If, on the other hand, we have a pantheistic God who is so completely interwoven with man and creation that creation is the sum total of God and vice versa, then how are we to clearly distinguish his voice and revelation apart from competing voices in the world? Even more, if God is creation and humanity, then wouldn't that make God both the cause and victim of humanity's woes? To answer the questions about our own existence, we need a God who is not created and yet who is present with us, condescending and accomodating himself to reveal truth and to deliver us.
This is much like Martin Luther's saying that the gospel message must come to us from outside--it is an "alien" message--because it proclaims an "alien righteousness" belonging to Christ and bestowed from God through Word and Spirit. In our own experience and life we find no salvation, nor can we find our way to God or a sufficient explanation of the world, until the gospel dawns upon us from heaven.
*Systematic Theology, vol. 1, p. 64. According to Wikipedia, Tillich uses "sources" to refer to the Bible and Christian history, "the medium" being the collective experience of the Church, and the "norm" as the theological standards of the biblical message by which all experience is to be judged. I am not here endorsing Tillich as a uniformly orthodox Christian, by the way.
Tillich would probably deny that we can speak of God's "nature" or "being," since that would mean he himself has being, thus raising the question of "From where comes God's being? What sustains or enables his existence?" For Tillich, speaking of "being", "essence," or "existence" is a purely human or creaturely reality and cannot be applied to God.