The question whether freedom of religion requires permitting the slaughter of animals in accordance with Islamic rites – a question which was the subject of a decision by the German Federal Constitutional Court in 2002 –is an example of the general problem how the law and the constitution of a modern, secular society can deal with the Sharia, a theonomous law which claims authority by divine revelation. The genetic links between western christianity and the development of the foundations of modern society in Europe do not provide evidence of the incompatibility of Islam and modernity. But they do emphasize that the tensions resulting from the immigration of 3 million muslims in Germany are a great challenge for the constitutional order. A modern constitution of the Western European and North-Atlantic type, such as the german constitution (Grundgesetz), does not allow the dissolution of the tensions between secular law and individual religious autonomy unilaterally in favour of the secular law. This kind of tension is by no means unknown in modern society. Functional differentiation and individualization are core characteristics of modern society. But in view of the challenge by Islam and other challenges, such as ecological problems, we can ask if the »experiment in modernity«, the project of the Enlightenment, is destined to fail because of the dissonance and the lack of understanding between the partial systems of the society. This question is the starting point to outline evolutionary processes of integration in modern society. In this context the constitution proves to be an important tool of integration. Controversies about the scope of validity claimed by the partial systems are matters of constitutional law. The modern constitution is a specific form in which modern society describes itself as a unity insuring »practical concordance« between colliding freedoms and claims of validity. In the conflict of modern ethical principles of animal protection and Islamic rules of slaughter, freedom of religion demands respecting the Islamic communities’ own path to knowledge and authority, notwithstanding the possibility that the arguments and the results may be unacceptable from the point of view of modern, scientifically based thinking. That is the crucial point in the decision of the Federal Constitutional Court. On the other hand, the constitution demands to be strictly respected in its core principles. Both freedom and respect for the core principles of a modern constitution form a basis for a social learning process which can contribute to a productive relationship between Islam and modernity.