Contrary to what many believe, secularism is not the denial of religion and it role in human life, it is just a separation between religion role in one's personal life and politics. A healthy society cannot function without ethical members observing morals and social rules. Religion provides a rich source of guidelines helping ethical behavior. Many studies have shown that religious people live a happier, healthier and more fulfilling life.
Nevertheless, religion should not be part of politics in a democratic society. Good political dialogue is based on free exchange of idea and free debate, thus any ideas are fallible and nothing in politics is scared. The use of religion will either lead to questioning religious symbols and dogmas, thus debasing and humiliating them, or will limit political debate for respect of religion. Putting people faith to questions at least anger people that may evolve in to sectarian conflicts and civil wars. Limiting political debate weakens democracy and the free exchange of ideas.
Applying religion as a source of legislation puts a group of small elites like Sheiks, Priests or Rabbis in charge and gives them absolute power. One can freely question ideas and laws proposed by politicians, but, it would be impossible to question someone claiming to represent God. One can hold humans accountable for their actions, but it is more difficult to question representatives of the Lord. This leads to oppression in his Almighty's name, since people with absolute power tend to abuse it. On the long run, society not only revolts against abusers of religion, but also against religion itself. Many Europeans countries that had Christianity imposed on them, have very militant anti-faith laws; for example France banning religious symbols in schools.
In addition, a good society should tolerate religious diversity and respects the freedom of faith for all its citizens. Hence, one faith could not be a source of legislation for a whole nation, because, it imposes religion of the majority on the minority. Some try to solve this problem by having multiple faiths as source of law, for example having Christian law apply on Christians, Jewish canon on Jews and Islamic Shariah on Muslims. Sudan is experimenting with this idea by applying Shariah on Muslims only and it is causing lots of problems. This is not only impractical, but also against the legal principle of uniformity within the law. It goes against fairness to cut someone's hand for stealing and put another in prison for the same crime.
Finally, religion is an individual choice and should be left to the individual. Democracy is rule of the majority by consensus, thus it is wise to separate between God's domain in the individual conscious and Cesar's domain of politics.
