This chapter will examine the history and theological debates of leaving Christianity and Christian faith. Throughout the history of Christianity, debates on who is a Christian, heretic, and an apostate have shaped the identity of Christians, and the power of Churches and rulers. After the Reformation and the Enlightenment, the ideas of secularism and liberalism, combined with recent developments of individualism (and) linked with various events, such as ethical debates on sexuality and gender, have resulted in a decline in Christianity in Western World. However, Churches and theologians disagree on whether to consider a leaver to be an apostate irrevocably, or should salvation persevere.