God is something often left undefined or poorly defined. Who is God? The Christian idea of God resulted when a whole load of mythology from the Egyptians, the Greeks and Romans, the Jews and many others got mixed together.  Jesus is God's son, no he isn't, he's God, unseparated from the Father. Are you confused? See Trinity to try and sort it out.
How Christians describe their god
If that were true, then the Bible would contain wonderful scientific information, excellent moral teachings, superb everything. In fact the Old Testament contains mythology from the Bronze Age; later there's Iron Age mythology; and after that in the New Testament there's Roman Mythology. The Bible is clearly the work of human beings who knew no more than other people living at their times in history. 
Extraordinary, miracle-working god
Christians are sure that their god must be real because he did wonderful miracles. He did stuff like causing a burning bush that Moses saw; Jesus healed the sick, turned water into wine, and did other spectacular stuff that only a god could do.
- How do we know that these miracles happened?
- Christians imagine that they know because the Bible says so!
- How do we know that the Greek myths are just legends?
- We know because the Greek myths say crazy, impossible things like the brothers and sisters of Zeus staying alive inside the stomach of their father.
- How do we know that the Bible stories are just legends?
Totally ethical god
Some abrahamists claim that their god's moral laws surpass human laws in supreme perfection, yet their god is comfortable with slavery, and with marking and disfiguring slaves, and with children being born into slavery, and separating slave families, and their god insists on circumcising boy and man slaves.  See Christian morality for more about ethical godliness.
- Why can't we assume that the forces of science caused the universe?
It has to be a god, or else Christians wouldn't have a reason to believe. 
Kind reliable god
Christians think that their god is good, kind, and loving, and that we can trust their god because he's just so good, kind, and loving.
- If their god is so good, kind, and loving, yet omnipotent, then why in the name of all that is holy do so many bad things happen?
We can't really judge God because God is too great for us. We must just believe that God is good and kind and loving despite all the bad stuff that happens.
- If their god can do so much bad stuff, or allow so much bad stuff, despite being good, kind, and loving, then their god might break promises despite being good, kind, and loving. If he promises to save us, then we can't trust that promise.
We have to trust that God will keep promises because ..... because err we must.