We all have specific presuppositions that influence how we view the world. Some might suppose that the world came about by entirely naturalistic processes without the need or existence of a God. Others might not consider the idea of theism at all and look at the world with a central belief that all people are equal and should be treated as such apart from any driving force from religion of faith. In Christianity, the central presupposition, the central thing that it claims to be true is that God exists. Indeed this is the central premise of all theistic religions, but especially the monotheistic Abrahamic religions. Who then is this Christian God? Can we, humans, ever understand him? And if we can, then what is God like?
Christians often rush at the first question to say “yes! We can know God” but I would like to tell all those that read this, that no, that’s not a complete picture of who the Christian God is. Indeed we can never have a full picture of this God within our finite minds. I have stated in the previous post that God is the necessary being upon which all things in this world are contingent. This implies that he is no mere thing, for anything plus another is something greater. Anslem put forth the ontological argument for God’s existence which, roughly, states that God is that which no greater can be conceived. This very nature of God as something beyond all thought and reason and natural evidence (for all natural evidence is contingent on the creative power of God) brings us to the central pillar of the Christian theology: God is incomprehensible.
God is incomprehensible to humans because, by necessity, if we could comprehend him in fullness, he would not be God. This reasoning backs up what is shown to humanity in the Bible. God is called ‘unsearchable,’ ‘inscrutable’ and ultimately that our thoughts are not his thoughts, and our ways are not his ways because his ways are higher than our ways, and in being higher, incomprehensible to us people . However, this incomprehensibility isn’t always a bad thing ndeed, it is because we recognize that God is incomprehensible that Christians can rest solidly on things they do not understand including some of the central thoughts of the Christian faith: the nature of Jesus, the unity of God in trinity, and the nature of our salvation. Indeed, God keeps secrets from humans, in that he acts as the sovereign father in wisdom protecting his children from harm . For Christians, this incomprehensibility of God is deeply humbling in that, much as the scientist opening the door to the natural world, theology can never fully know God and that there is always more about him to know.
However, this is not to say that we cannot know God. The second major presupposition, the second major truth the Christians should proclaim always, is that we can know God and that we can know him truly, personally, and sufficiently . We know him truly in that while we can know him fully, we can know him truly in that while incomprehensibility leads the Christian to conviction in matters which are beyond human comprehension, the knowability of God leads the Christian to conviction in the nature and character of this God. Indeed, the knowledge of God is the center of joy for a Christian  and it is the basis of our hope for the future  and it allows us to love other people with the same love that God shows to humanity .
So of what character is this God Christians worship? While the scope of this question is too large for any single post, I find one of the greatest summations of the Christian belief of the Character of God in the historical church document, the Westminster Larger Catechism. Indeed when we ask ‘What is God’ it tells us:
God is a Spirit, in and of himself infinite in being, glory, blessedness, and perfection; all-sufficient, eternal, unchangeable, incomprehensible, everywhere present, almighty, knowing all things, most wise, most holy, most just, most merciful and gracious, longsuffering, and abundant in goodness and truth. 
This is the God that Christians believe in, a God infinitely worthy of praise and glory, a God sovereign over all things, immutable, and (quite simply) awesome. This is not a God of mere weakness, but one that is strong beyond all measure, and loving with abounding steadfast love . This is the God I worship, and the God I seek to understand, and the God I wish all people to know and love also. Finally, to return to the Nicene Creed this is the God that is Almighty, maker of heaven and earth and of all things visible and invisible.
(Post 1b in a series explaining Christian theology in the framework of the Nicene Creed)
 Psalm 145.3, Romans 11.33-34, Isaiah 55.8-9
 Deuteronomy 29.29
 2 Peter 1.3
 Jeremiah 9.24, Galatians 6.14
 1 John 1.1-2
 1 John 4.7
 Westminster Larger Catechism, Question 7
 John 4.24, Exodus 3.14, Job 11.7-9, Acts 7.2, 1 Timothy 6.15, Matthew 5.48, Genesis 17.1, Psalm 90.2, Malachi 3.6, 1 Kings 8.27, Psalm 139.1-3, Revelation 4.8, Psalm 147.5, Romans 16.27, Isaiah 6.3, Revelation 15.4, Deuteronomy 32.4, Exodus 34.6
 Exodus 34.6
The Westminster Confession of Faith gives a longer discription of the Christian God that I do so like.