Why don't Christians believe in one God?

To all of you who think There is more than 1 God (Jesus & holy spirit) this is the biggest sin a man can commit. There is only ONE God. Jesus was a prophet from him. And know that YOU can not skip the judgment day b/c u think you can cheat it by having someone else buy your sins. And if the only way to heaven is through Jesus, then God will NOT be a merciful God to not let 5out of 6 people into heaven. there are only about 1 billion Christians and 6 billion people on earth.