CCELA free digital library of hundreds of classic Christian books selected for edification and education. The CCEL includes searchable online versions of the Ante-Nicene Fathers, the Nicene & Post-Nicene Fathers series as well as works by many others across church history, including Catherine of Siena, Martin Luther, John Wesley, Kierkegaard and more. Look in the Home/Church Fathers or Browse/Author sections to start looking for particular theologians. The editions are in the public domain. Past Masters should be consulted for any works duplicated there in newer editions, eg Augustine and others. The interface is a bit clumsy, it has ads, but it provides a lot of useful primary source texts.