I just want to note some resources for the study of Christian identity as resident aliens, especially with its reference in the Epistle to Diognetus:
1. Stanley Hauerwas and William H. Willimon. Resident Aliens: Life in the Christian Colony. Nashville, TN: Abingdon Press, 1989.
2. Stanley Hauerwas and William H. Willimon. Where Resident Aliens Live: Exercises for Christian Practices. Nashville, TN: Abingdon Press, 1996.
3. Benjamin H. Dunning. Aliens and Sojourners: Self as Other in Early Christianity. Philadelphia, PA: University of Pennsylvania Press, 2009, pp. 64-77. (Preview at Google Books)
4. George Weigel, The Church’s Political Hopes for the World; or, Diognetus Revisited, in Carl E. Braaten and Robert W. Jenson (eds.), The Two Cities of God: The Church’s Responsibility for the Earthly City. Grand Rapids, MI: Eerdmans, 1997, pp. 59-77. (Preview at Google Books)
5. George Weigel. Soul of the World: Notes on the Future of Public Catholicism. London: Gracewing Publishing, 1996, pp. 31-46. (Preview at Google Books)
6. William L. Portier, Heartfelt Grief and Repentance in Imperial Times, in David L. Schindler (ed.), Love Alone is Credible: Hans Urs von Balthasar, as Interpreter of the Catholic Tradition. Grand Rapids, MI: Eerdmans, 2008, pp. 349-360. (Preview at Google Books)
7. Bruce W. Winter. Seek the Welfare of the City: Christians as Benefactors and Citizens. Grand Rapids, MI: Eerdmans, 1994. (Preview at Google Books)
For a good translation of the Epistle to Diognetus (which is, otherwise, readily available on the internet), you could refer to this book:
1. Bart Ehrman. The Apostolic Fathers, Volume II: Epistle of Barnabas. Papias and Quadratus. Epistle to Diognetus. The Shepherd of Hermas. Cambridge, MA: Harvard University Press, 2003, pp. 121-159. (Preview at Google Books)