Apologetics (from the Greek apologia) is a mode of religious discourse aimed at persuading or convincing those of other faiths or none as to the truth of those religious claims by providing philosophical, theological and sometime existential or rhetorical reasons for accepting those positions.
Early apologetic literature in the Christian tradition was aimed at convincing Jews as to the role of Jesus Christ and responding to the heretical views of Platonists, gnostics and pagans. The writers of these works include the Church Fathers Justin Martyr, Origen, Tertullian and St Augustine. In the Middle Ages, apologetics became more scholarly, with St Thomas Aquinas and others writing extended philosophical discussions on matters of faith.
The dawning of the modern age brought with it a great number of topics that caused apologists to respond: the rise of Protestantism brought with it apologetical literature on both the Protestant and Catholic side, modern science including the discoveries of Copernicus and Charles Darwin, as well as rigorous and even radical Biblical criticism, with the publication of works by David Strauss, Thomas Paine and Albert Schweitzer challenging the religious status quo.
Modern apologetical literature tends to be either of an academic vein, of the sort given by John Haught, John Polkinghorne, Alister McGrath and Richard Swinburne, or a more conservative, populist type of apologetics intended for a general reader - paradigmatic examples include Lee Strobel's The Case for Christ and Rick Warren's The Purpose Driven Life. There is also an emerging category of counter-apologetics - publications aimed at criticising or debunking apologetical literature, or making a case against religious ideas.