The parts of the religion concerning the faith of the individual which remain hidden in his heart and cannot be seen, are called the roots (in Arabic, Usool) because they are its fundamental beliefs – just as the roots of a tree which remain hidden under the earth cannot be seen, yet they are the tree’s very foundation. A true believer must ensure his/her faith is firmly grounded in the correct knowledge and understanding of these fundamentals (Usool-e-deen), and then keep them constantly nurtured and cultivated to allow for the healthy growth of its branches, the Furoo-e-deen (Branches of Religion). There are 5 fundamental articles of faith, or Roots of Religion, that every Shi’a Muslim must research and properly understand before truly believing:
1. Unity of God (Tawheed)
2. Justice of God (Adaalat)
3. Prophethood (Nabuwwah)
4. Successorship of Prophets (Imaamah)
5. Day of Resurrection (Qiyamah)
The above summaries are taken from the book Invitation to Islam, by Moustafa al-Qazwini.