There is no doubt that secularism contradicts Islaam in every aspect. They are two different paths that never meet; choosing one means rejecting the other. Hence, whoever chooses Islam has to reject secularism. In the following, we go in the details of explaining why.
Secularism makes lawful what Allaah has made unlawful.
The Rule of Allaah (Sharee'ah) is compulsory and has basic laws and regulations that cannot be changed. Some of these laws are concerned with the acts of worship, the relations between men and women, etc.
What is the position with regard to these laws? Secularism makes adultery lawful if the male and the female are consenting adults.
As for Riba (interest on money), it is the basis of all financial transactions in secular economies. On the contrary, Allaah says (which means) "O you who believe, fear Allaah and leave what comes from Riba if you are believers. If you do not do so, then wait for a war from Allaah and His Messenger." [Surah Al-Baqarah (The Cow) Verses 278 and 279]
As for alcohol, all secular systems allow the consumption of alcohol and make selling it a lawful business.
Secularism is clear unbelief (Kufr).
Secularism is based on separating religion from all the affairs of this life and hence, it rules by law and regulations other than Allaah's laws. Hence, secularism rejects Allaah's rules with no exception and prefers regulations other than Allaah's and His Messenger's . In fact, many secularists claim that Allaah's laws might have been suitable for the time they were revealed but are now outdated.
As a result, most of the laws governing the daily affairs of life in the countries ruled by secular systems contradict Islaam. Allaah says (which means) "Do they seek a judgment of Ignorance? But, who, for a people whose faith is assured, can give better judgment than Allaah?" [Surah Al-Ma'idah (The Table Spread with Food) Verse 50]
Ibn Katheer said in the Tafseer of this verse that Allaah is denouncing those who reject His ruling and accept other rulings that are not based on the Sharee'ah of Allaah. Whoever does so is indeed a non-believer. Indeed, belief in Allaah can never go with the acceptance of other than His rulings in one's heart. Allaah says (which means) "If any do fail to judge by what Allah has revealed, they are non-believers." [Surah Al-Ma'idah (The Table Spread with Food) Verse 44]
From the above, the status of secularism and its relation to Islaam are clear. But the ignorance about the Islaamic truth is still dominating the Muslim's mind. Most secular systems repeat slogans like "no religion in politics and no politics in religion" or "religion is for Allaah, and the state is for the people." Such sayings portray their view of Islaam as a religion to be practiced in the mosque only, and that it should not be allowed to rule life outside the mosque. Furthermore, they try to deceive people with democratic slogans like "personal freedom" and "people governing people." That means that people come first and no place is made for the ruling of Allaah.
This is why secularism is clear Kufr, this is why secular systems have no legality and authority and should be rejected by Muslims.