People like Charles Darwin, Sigmund Freud, Hegel and Karl Marx viewed man from various angles. In their search for understanding human nature, some concentrated on the body and its form and shape. So they came to certain conclusions which were body-centered. Some viewed man from the occupation he once followed, and thought of him as an animal subjected for evolution. The thought of some other thinkers fell on human belly and they related everything with it. They found it most important and decided that hunger was the cause of all problems. They thought that once this problem is solved, everything would be alright. Those who viewed man and life through sexuality believed it to be the most important thing and whatever they knew, saw and heard was related to it. They declared that the whole human activity is sex centered and if the restrictions imposed on sex were removed everything would be all right. Some others gave undue importance to spirituality and disregarded the body and its needs. So the people who tried to understand man in the absence of divine revelations were like the proverbial blind men, who tried to understand the elephant in their own partial ways. They arrived at certain conclusions which were imperfect and full of errors.
Those who do not know man cannot understand how man should live. Every individual must possess the basic awareness to understand how he should live in this world. This awareness is achieved only through the divine message. It was religion that taught man about the meaning, objective and destiny of his existence.
The manufacturer knows best about the product he manufactures. To give an example, vehicles work best with the oil, spare parts etc. recommended by the vehicle manufacturer. Even if it is a simple thing like air pressure of the wheels, we follow what the manufacturer says. One might ask why? It is our vehicle. We own it and we use it. Why not use it the way we want? Why not change the air pressure of the wheel depending on our mood that day?
We would not try changing it based on our whims and fancies, because we trust that the vehicle maker knows what is best for the vehicle. We trust the manufacturer of the products even for trivial things, how is it rational to make decisions about our lives without looking for guidance and instructions from our creator, God Almighty?
If I may call human beings a machine, we are the most complex machine on earth due to our emotions and thoughts. Our creator, God, knows exactly what is good for us and has prescribed a way of life which was taught to us by the Messengers of God and the scriptures revealed by Him. Like as The Holy Book Quran revealed in the time of Mohammad The last Messenger. Going away from the guidance of God, would lead to chaos and destruction.
Since Islam was revealed from our creator, God Almighty, it is a complete way of life and therefore addresses all the aspects of human life.
Indeed, the religion in the sight of Allah is Islam.