Tom (NT) Wright – Simply Christian
Tom (NT) Wright – Simply Jesus
C. S. Lewis, Mere Christianity (London: William Collins, 2016)
Although this was written in the 1940s, it remains a very readable introduction to what most Christians believe.
Jonathan Hill – The History of Christian Thought
An up-to-date introductory text.
Augustine of Hippo, The Confessions. https://www.ccel.org/ccel/augustine/confessions.pdf
Augustine's Confessions is a very accessible (ancient) text dealing with the nature of faith.
Thomas Aquinas. Summa Theologica. http://www.newadvent.org/summa/
Aquinas is perhaps the most prominent theologian in Christian History; he was a prolific writer, and his Summa Theologica/Theologiae is one of the most comprehensive attempts at a systematic Christian Theology. It is linked here, in its entirety. Speak with your teacher about specific "questions" as Aquinas calls them, which may relate to your interest. Good starting points for understanding Christian Theology are Questions 2-13 of the Prima Pars (first part).
Diarmaid MacCulloch. A History of Christianity: The First Three Thousand Years (2009)
Diarmaid MacCulloch. Reformation: Europe’s House Divided 1490-1700 (2003)