This will be one of the simplest and shortest straight-to-the-point answer you will ever find! This question is one of the most asked questions by the atheist community throughout the world with unbelief to the existance of God.
Quoting from Ethics Internet Encylopedia of Philosophy, British Pilosopher David Hume argued that "moral assessments involve our emotions, and not our reasons." Furthermore, "Ethics applies asmuch to behavior of the atheist as to that of devouted religious person." Oh, mind you, the founders of philosophy were not devoted Christians... So why do human beings both devoted Christians and atheist "FEEL" deep down within their hearts that it is unethical, immoral and wrong to kill, rape, etc...? Why does morality and immorality exist? How can we explain their existence?
Firstly, The Holy Bible clearly states the existance of a God. This God is the Creator of the entire universe-especially planet earth considering Genesis 1:1. This God is a God of Love according to 1 John 4:8-His principality and character. His Law in Exodus 20 is based on His Principality and Character.
So why believe The Holy Bible? Well, The Holy Bible is God's Word to us, mankind or humanity. Meaning, the Word inspired certain human beings according to His Will in the past-the message from the Word to the people during that time of these certain inspired individuals, what actually happened, etc.., is all written down in The Holy Bible. The Word according to John 1:1-5 created everything that was created in the beginning in Genesis 1 & 2-this Word is the only Truth according to John 14:6, and His Name is Jesus Christ. This Word even made it clear that only those who believe in Him (The Word, furthermore, what The Word clearly states in The Holy Bible) should not perish but have eternal life, according to John 3:16!
Face it, if you're an atheist who doesn't believe in the existance of God, I would like you to ponder upon the following questions... If God or Love (1 John 4:8) does not exist, then why do you as a human being desire to be loved by someone else or to love someone else in general? Why do you feel that you love your parents, your kids, your wife, or your husband? If Love does not exist, why is it wrong to kill someone, steal, etc...? What makes you feel its wrong to kill someone, steal, etc...? What did you thing happen to the minds of the individuals who made our societies laws to think that its wrong to murder, steal, etc...? Why do we (globally speaking) have laws that is like a line that seperates morality from immorality or from right from wrong? Why do you feel morality and immorality or right and wrong exists at the first place?
See my friends, because God (Love in 1 John 4:8) exists you feel its wrong to murder, etc.., considering the fact that we (mankind) were created in the Image of God in Genesis 1:27, Who is Love in 1 John 4:8! Morality and immorality eixsts due to The Great Controversy which started in heaven considering Revelation 12:7-this Great Controversy is is now faught in this earth in which mankind fell according Genesis 3... Right and Wrong, Morality and Immorality exists because God exists, and so is the devil. Anyways, every choice has a consequence-eternal life or death will depend entirely on your personal choice: "believe at your own choice!"