When the government is in the hand of only one man, he is called king.
In Medieval Times the king owned the land of his kingdom who divided it among his local Lords. They in turn would parcel out land to lesser vassals who would have serfs work for them. In return, the Lords would collect taxes for the king and provide knights who would fight to protect his kingdom. The absolute monarchy arose because the Reformation had lead to cruel wars of religion and states erupted into civil wars. So, more and more power passed into the hands of the king. He collected taxes directly, raised his own army and brought the judicial system under his control. The power of the king did not rest only on wealth from taxation and military power but also on the "Divine rights of Kings" which Bishop Bossuet espoused.
Bishop Jacques-Benigne Bossuet (1627-1704) was a French bishop highly respected at the court of Louis XIV of France. He elaborated the Medieval notion of the "Divine Right of Kings". He based his teaching on God's biblical choice of kings such a Saul and David realizing that even if the European kings were not anointed, they were still sacred as representatives of God's majesty. The power to rule comes from God and the king's majesty is the image of the greatness of God in the prince. The king incorporates in himself the will of the entire people of his kingdom and his Royal power extended throughout his kingdom and keeps it in order.
However, the bishop also warns the kings that they must rule in a noble and benevolent manner because "You are children of the Most High; it is he who established your power for the good of humanity. But, O gods of flesh and blood, O gods of mud and dust, you will die like men; you will fall like all the great. Greatness divided men but briefly; a common fate finally renders them all equal." (Webpage: Bishop Jacques-Benigne Bossuet - The Divine Right of Kings)
Could a king be deposed?
Bossuet states, "The person of the king is sacred, and to attack him in any way is sacrilege." This meant that "No group, whether they be nobles, or a parliament, or the people in the street, have a right to participate in this rule; to question or oppose the monarch was to rebel against God's purpose." Of course, if the king commanded action contrary to God's law you refused but had to suffer the consequences. However, long before Bossuet St. Thomas Aquinas (1225-1274) had written, "If to provide itself with a king belongs to the right of a given multitude, it is not unjust that the king be deposed or have his power restricted by that same multitude if, becoming a tyrant, he abuses the royal power." Thomas Aquinas, On Kingship, in Din Bigongiari, ed., The Political Ideas of St. Thomas Aquinas, New York: Hafner Press, 1953, Book I, Ch. Six, 49-51.
The Divine Right of Kings had already reached beyond it limits when Henry VIII (1491-1547) made himself the head of the Church in England thus usurping the spiritual power or the Pope.*
* "In a draft justification of the king's proceedings which survives in the Public Record Office, Henry is said to have taken the title of supreme head `following the right of the kings in Judah and Israel', ...."(Webpage: Rex-HenryVIII/Richard Rex, Henry VIII and his Church)
Theory of Social Contract