Montesquieu's work represents a great deal of effort and time on his beliefs in political Enlightenment ideas. For example, the idea of separation of powers for government to be divided into different branches to allocate the power to multiple arenas help to minimize political tyranny or despotism. Another idea that comes out of the work is the idea of check and balances, that the different branches of government has the ability to limit the power of the other branches. The work also espoused the need for individual liberty and a rights based political order, critically essential to the Enlightenment period.
Of course, with a book this big, there are many important points in this book. I would say, however, that the most important idea or ideas in this work is the idea of a limited government that is set up in a way that prevents it from abusing its citizens.
The ideas of Montesquieu can be seen quite clearly in the Constitution of the United States. In The Spirit of the Laws, he talked about the need for the rights of individuals to be protected. He argued that the best way to do this was through a written constitution that guaranteed these rights. He also said that a government based on separation of powers was needed to ensure the rights of the people.
One of the central ideas of The Spirit of the Laws is that countries' governments correspond to their principles. For example, democratic countries are motivated by the love of virtue, of placing the needs of the greater good above individual interests. People under a despotic government are guided by the principle of fear, and those under a monarchical system of government are guided not by virtue but by honor, the desire to attain titles and privileges from the monarch. Therefore, the three underlying principles in different forms of government are virtue (in a democracy); honor (in a monarchy); and fear (in a despotic government). The educational systems in each form of government will have different goals--virtue in a democracy, fear in a despotic system, and honor in a monarchy. These different systems also have different types of laws and different treatment of women (for example, in monarchies, dowries are large to support one's wife according to her rank, while they are moderate in democracies and non-existent in despotic systems in which wives are considered the property of their husbands), as societies are guided by their overarching principles in making laws.