Many contemporary scholars would balk at the suggestion that a comparative study of religions is even possible. The suggestion rests on an assumption there exists a common thread working its way through the world's great (or not so great, depending on your perspective) religious traditions. That assumption holds the thread can be drawn out of all religious traditions, including Buddhism, Confucianism, Hinduism, and also Judaism, Christianity, and Islam, without doing significant violence to the tapestry of any one tradition. A tall order, to be sure. Maybe even impossible to complete, since each religious tradition is itself host to a wide varieties of expressions within its boundaries, a variety that has sometimes even been generated through an encounter with another great religious tradition.
But is it, in principle, impossible? In other words, are the great religious traditions essentially incomparable, even if superficial points of comparison might appear? Note carefully what is being asked. Were they essentially incomparable, this would mean anyone who got themselves stuck in a way of thinking that they were comparable would judged to fall between being intellectually naive and morally reprehensible. (Damn colonizer! colonizing the world with your intellectual discourses...) The stakes are naturally quite high.
Those familiar with Judaism, Christianity, and Islam, for example, might be tempted to make the following observations. Buddhism has no supreme God. Does it count as a religion? Hinduism has so many gods it has been confused with Greco-Roman polytheism. Does it count as a full-fledged religious tradition? Confucianism looks more like a social ethic, holding to a vaguely Socratic sentiment about why one would want to know about the gods and the afterlife if you do not know your own self. Does it count as a religion? This line of questioning loads the dice in favour of certain traditions over against others. Those traditions that profess belief in a single, exclusive, supreme deity are judged superior over those tradition that do not. This is a sure way to be accused of intellectual mischief, if not by your peers, then by your successors. Not a few Western, and usually Christian, scholars have fell under this condemnation.
Over on the Eastern side, the supposed exclusivity of the Western traditions is particularly problematic. Is divine truth so mundane that it can picked up and tossed about with ease by mere human beings? Does it reflect a pious disposition towards divinity for a person to seize hold of some "truth", some statement, some intellectual formula, and assert it as divine truth? The divine order presumably doesn't speak a human language; it also remains a question whether it speaks, in an active sense, at all.
The lesson I believe it is important to keep in mind is that the best way to walk through a minefield is with one's eyes open. (Let's set Confucianism aside, since it introduces a rather different set of considerations, to focus on two of the great religious 'families'.)
The Professor of Comparative Religion, who I have worked with now for a few years, Dr, Arvind Sharma, proposes a very general rubric to compare Abrahamic monotheism and Indic monotheism. The later term he stretches to include Hinduism, Jainism, Sikhism, and also, surprisingly, Buddhism.
The comparison proceeds as follows: Abrahamic monotheisms generally hold that God is distinct from the world of human experience, on account of being its Creator; and holds that all other gods are false gods. The Ten Commandments ("You shall worship no other god..") and the Shahadah ("There is no god but God...") both bear out the general characterization. Compare this to Indic monotheisms, which tend to hold that God and the world are ultimately One, and holds that all other gods are ultimately manifestations of that One. Hinduism holds to the ultimately unity of all things ("Brahman is Atman"; the soul is all things), while Buddhism is merely its metaphysical inversion ("Nirvana/Anatman"; no soul is in no thing, or all things are empty.)
Here we seem to have a common thread working its way through a number of religious traditions: an admonishment to not let the "external", material things (wealth and possessions, the praise of others, authority, etc.) cloud one's judgment. The Abrahamic traditions caution against fashioning of gods for oneself, while the Indic traditions peel back the veil covering over the 'relative' worth or 'emptiness' of the same. The messages are roughly approximate; they regard knowledge of one's 'self'. Should you have in your head that your accomplishments, natural attributes, possessions, or some other type of personal accessory elevate you above your fellow human beings, you are a sad, sorry, sack of...foolishness. Our common end, in bodily death, lays bare all that is pretense and posturing.
I make some observations, by way of clarification, about Dr. Sharma's very general taxonomy of religion. It seems to me that what is being compared are not differing conceptions of God or divinity, but differing conceptions of human nature--whether there is one, what it entails, and so on. What is not being described is an utterly transcendent divine order; but rather the relation between a divine order and humanity, or more specifically, how human beings understand themselves in relation to the divine order. Or, even more specifically, human self-knowledge.
What I mean to suggest is that there is a very general, and essentially comparable, difference between how Abrahamic monotheisms and Indic monotheisms conceptualize human life in time. Abrahamic monotheisms tend to hold that time is finite, with beginning and end, and also that human beings gets a single chance to live their life on God's green earth. On the other hand, Indic monotheisms tend to hold that time is an infinite, endless circle, and also that human beings go around and around the karmic wheel of birth, death, and rebirth. In both cases, the human being is the same human being we are all readily acquainted with (being human beings ourselves). But the nature of human life, where it comes from before birth and where it goes after death, is radically different.
Hence the difference, it also seems to me, between a Abrahamic understanding of salvation and a Indic understanding of liberation. If one understands oneself wholly cut off from the divine order, which is implied in conceptualizing God as Creator, it will take divine aid to 'return' to a right relation with God. Whereas if one understands oneself as an immediate participant in a divine order, which is implied in conceptualizing God as ultimately the same as the world, it is pointless to speak of divine aid in the process of achieving liberation. The human being is, always has been, and always will be God.