Who is the main subject of the holy quran?

A call for the whole mankind to respond to God's message which was repeatedly conveyed by all of the prophets through history & finally by the last prophet Muhammad pbuh. This message teaches & stresses the oneness of God, Allah the Creator of everything, the Lord, the Master of the day of Judgement, the only one true God with none associated to Him at all & consequently we are commanded to obey & worship Him alone.