"Why do christians believe in jesus being god"

For the same reason that muslims believe that the prophet had a revelation from the Angel Gabriell in a cave, did a night flight to Jerusalem on a horse and ascended into heaven to negotiate with Allah about how many times a day to pray.
and
that hindus believe that the world is carried aloft on the back of a giant turtle
and
that Australian Aboriginals believe that your earthy existence is not real and that you are just part of the creators dream.

And why an atheist does not believe in any of these as no convincing proof of such things has ever been brought forward.

Simple really

"Deaths in the Bible. God - 2,270,365
not including the victims of Noah's flood, Sodom and Gomorrah, or the
many plagues, famines, fiery serpents, etc because no specific numbers
were given. Satan - 10."