Christian Approaches to Other Faiths (London, UK: SCM, 2008), edited by Alan Race and Paul Hedges, is a good place to start exploring for answer. The book is divided into two parts. First part is on theoretical and methodological issues while the second part on Christian responses to various religions.
Alan Race, Dean of Postgraduate Studies at St. Philip's Centre, begins the book with his essay 'Theology of Religions in Change: Factors in the Shape of a Debate.' One factor is Christians' interest in other religions. Race lists three reasons for such interest.
First, Christians have a mission to reach out to everyone. Hence it is important for Christians to learn how to relate to other religions, whether is there a need to evangelize; if yes why so, if not why not? Second, religious extremism in our time poses a serious threat to everyone. Christians need to discern how should religion, theirs and others', be expressed not as threat but for human flourishing. Third, for the sake of theological truth. If there are other religions around, then how can Christians give an account for them? (pp.5-6)
Then Race moves on to the next factor, which is on the sources for our reflection on other religions. He cautions the use of scriptures and tradition when we theologize about the religious other. Our interpretation and application of the scripture and tradition cannot be the "sole determiner" or "final arbiters" for our theology of religions. (pp.7-8) Race's suggestion here is helpful but left us hanging. If scripture and traditions are not the only determiners of theological judgements on other religions, then what other sources can we draw from to produce a view that is distinctively Christian?
In the next section, Race provides three areas of interest for us to explore in relation to religious plurality. The first area is on how religious plurality shapes the meaning of our understanding of Christian belief. For example, our knowledge of Jesus Christ as our Lord and Saviour needs to be understood vis-a-vis other religions. The second area of interest is in the development of Christianity itself, how can theology be enhanced, without being relativistic, through its interaction with other religions? The third area is in inter-religious dialogue, particularly in the converge or similar ideas found in different religions. (pp.9-11)
Race ends with a call to negotiate between "all the same" and "all different" as a way forward to find a middle path to construct a Christian theology of religions. He points out that this tension is reflected in the New Testament, such as: "Whoever is not against us is for us," (Mark 9:40) and, "Whoever is not with me is against me." (Matt. 12:30)
What I found most provocative in this chapter is the probing question that Race asks: "If Christian theology is a process of reflection on experience---as in the famous Anselmian definition of theology as 'faith seeking understanding'---then we might ask about what constitutes the data of experience.... What level of impact might the data of other religious experiences and convictions have?" (p. 9)
The answer to this question is, I think, the key to an appropriate theological account of every other religion and the reality of religious plurality.