Philosophers and theologians have compared various religions to elucidate both their similarities and their differences. Today, though, there is something called global theology or world theology which is the attempt to compare two or more religious traditions from the perspective of one in particular. Also sometimes called comparative theology, it is often a self-consciously partisan enterprise pursued by theologians with specific faith commitments (and agendas).
A more disinterested and objective form of comparative theology may be impossible. Anyone doing theology, rather than religious philosophy, will already have strong religious commitments to certain faith propositions. They will normally write within a context of specific religious authorities, texts, and traditions. A more objective comparison of various theologies would require setting all of that aside, something that can be difficult even for someone used to doing so.
There are risks involved with the pursuit of comparative theology. A person attempting to study one religion from within the context of their own theological tradition will necessarily bring with them the categories and concerns of their tradition, many of which may not be relevant within the other traditions being studied. Attempts to apply them to other traditions despite this can quickly lead to misunderstandings and errors.
Any form of comparative theology must begin with the acceptance that religious pluralism is a fact of the modern world and that this pluralism might allow any one tradition to shed some light on others. Traditional theology has typically assumed the primacy of one theological tradition and ignored or denigrated others. In comparative theology, though, the recognition and study of religious pluralism is a primary point.
In some cases, the label global theology is used because those involved are attempting a generalized effort to develop a theological system or understanding that would apply to all religions (or at least all major religions) around the world. Thus, a common hope is to extract theology from a specific religious tradition and remake it into a more generalized academic discipline. At the same time, though, scholars involved remain committed to the faith doctrines of their own tradition.
To accomplish their goals, those involved with global theology have needed to first and foremost disconnect the term theology from its roots. Technically speaking, there can be no theology of religious systems that have no gods (as is the case with some forms of Hinduism, Buddhism, and Taoism), so the term has had to be broadened to refer to any intellectual, philosophical reflection on religion and religious traditions.
Global theology, or perhaps it would be better to say global philosophy of religion, typically attempts to address a couple of specific issues: How does this religion describe the nature of reality, the universe, and the sacred/divine? How does this religion attempt to address the problems of the human condition? What sort of salvation, transformation, or transcendence does this religion offer its adherents?
In each case, the answers of one religion are compared to those of other religions in order to discover commonalities and thus allow scholars to describe something approaching a common human religion. Its not that the scholars are seeking to create a single religion out of all the traditions they find; rather, there is a general attitude of trying to foster greater understanding and tolerance among religions and the discovery of basic similarities is thought to be an important means for achieving this.
It is interesting to note that something very like this was already occurring among ancient philosophers. Theology originated among the ancient Greeks, and Stoic philosophers may have been the first to attempt a systematic comparison of various religions in order to identify commonalities. It was with Christianity that the dismissal of other religions became a cornerstone of theological work.