In The Name of Allaah, The Most Merciful, The Bestower of Mercy
Even though the words of the Adhaan are little, they gather affairs of Aqeedah within them; because the Adhaan begins with the proclamation of Allaah’s Exaltedness (or Greatness) and this necessitates Allaah’s Existence and Absolute Perfection; then extoling Allaah with Tawheed and negating a partner (or partners) from Him; then affirmation of Muhammad’s Messengership (sallal-laahu-alahi-wasallam); then a call to a specific act of obedience to Allaah preceded by the testimony to Muhammad’s Messengership, because this act of obedience cannot be known except through the Messenger (sallal-laahu-alayhi-wasallam); then a call to success and it is that permanent residence, and in it is an indication to the afterlife. Then what is repeated of the words of the Adhaan are repeated for emphasis. 
 Paraphrased statement of Al-Haafidh Ibn Hajar (rahimahullaah), see link: http://www.sahab.net/forums/index.php?showtopic=134568#entry683177
This faa-idah was sent by our brother Dawud [Markaz Mu-aadh (Slough)]. May Allaah reward him with good and grant all of us the goodness in this life and the next. Aameen.