The Origin of the Universe
Believers say God made everything - the story of the 6 days of creation in Genesis (accepted by Christians and Moslems) indicates that God made the universe.
Most believers would say that Genesis is a metaphorical account, meaning it does not give an eye witness description. But whatever did happen, God caused it.
"In the beginning, God created the heavens and the earth." (Genesis 1)
The Cosmological Argument - St. Thomas Aquinas taught that the universe must have a "First cause" - God. Based on observation of "cause and effect."
Non-believers say that the universe happened by accident, not God's will. They cite the Big Bang as giving evidence of this.
The Design of the Universe
Again referring to Genesis, Christians say the world is like it is because God designed it this way.
In particular, "God created human beings in His image." (Genesis 1)
"In his image" means humans are like God, having awareness, free will, and a sense of right and wrong.
The Design Argument, proposed by William Paley, says God must exist because the natural world appears to be designed. Things are complex and fit for a purpose. Paley explained this using his "watch analogy."
Non-believers often mention Darwin's Theory of Evolution as a scientific explanation of the complexity in nature. They believe that the chance process of Natural selection explains life on earth where only the fittest survive.
Non-believers may also object to the idea of God as a designer because of the random cruelty present in nature.
Many believers accept that the Big Bang and theory of evolution do enlarge our understanding of these ultimate questions. But they also feel that they can be seen as complementary to faith in God.