"And fear the Day when ye shall be brought back to Allah. Then shall every soul be paid what it earned, and none shall be dealt with unjustly."

Quran

 Imagine that the traffic code, for example, has been enacted without any penalties for the offenders. According to the traffic code in our country, you have to take the right side on streets while you are driving, you should have a driver's license, …etc. People will never abide by these codes if there is no penalty for those who violate them. Paying a fine of 5000 liras, retrieving license, vehicle seizing, putting into jail, …etc are all penalties imposed if certain driving violations occur. Hence, it's impossible for any law to be enforced if it is not accompanied with deterrents against whoever violates it. Similarly is believing in Allah; believing in Allah is the Creed, and believing in the Last Day is the deterrent against the unbelievers.