"Why did God create only one Adam and not many at a time?
He did this to demonstrate that one man in himself is an entire universe. Also He wished to teach mankind that he who kills one human being is as guilty as if he had destroyed the entire world. Similarly, he who saves the life of one single human being is as worthy as if he had saved all of humanity.
God created only one man so that people should not try to feel superior to one another and boast of their lineage in this wise: "I am descended from a more distinguished Adam than you."
He also did this so that the heathen should not be able to say that since many men had been created at the same time, it was conclusive proof that there was more than one God.
Lastly, He did this in order to establish His own power and glory. When a maker of coins does his work he uses only one mould and all the coins emerge alike. But the Kings of Kings, blessed be His name, has created all mankind in the mould of Adam, and even so no man is identical to another. For this reason each person must respect himself and say with dignity, "God created the world on my account. Therefore let me not lose eternal life because of some vain passion!"
Full text of the book is here