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.

Christian Theology

Augustine of Hippo, The Confessions

A very accessible text dealing with the nature of faith.

Thomas Aquinas. Summa Theologica

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).

Church History

Diarmaid MacCulloch. A History of Christianity: The First Three Thousand Years (2009)

Diarmaid MacCulloch. Reformation: Europe’s House Divided 1490-1700 (2003)

An essential starting point for an overview of Church History.