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.
So what is God exactly?
We're trying to work that one out but we aren't getting very far.
If that were true 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 God must be real because God did wonderful Miracles. God 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 God could do.
- How do we know these miracles happened?
- Christians imagine they know because the Bible says so!
- How do we know 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 the Bible stories are just legends?
Totally ethical God
God's moral laws surpass human laws in supreme perfection, yet God is comfortable with slavery. God is comfortable with marking, disfiguring slaves, God is comfortable with children born into slavery, God is comfortable with separating slave families. Still God insists on circumcising boy and men slaves.  See Christian morality for more about ethical Godliness.
- Why can't we assume the forces of science caused the universe?
It has to be God or Christians wouldn't have a reason to believe. 
Kind reliable God
Christians think God is good kind and loving, we can trust God because he's just so good, kind and loving.
- If God is so good, kind and loving why in the name of all that's 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 God can do so much bad stuff or allow so much bad stuff despite being good, kind and loving then God might break promises despite being good, kind and loving if he promises to save us we can't trust that promise.
We have to trust that God will keep promises because ..... because err we must.