In his brilliant little book, Drudgery Divine: On the Comparison of Early Christianities and the Religions of Late Antiquity, J.Z. Smith argues that the entire enterprise of comparing "Christianity" with religions of Late Antiquity has been, from its inception, not one of scholarly and academic pursuit, but rather one of apologetics. In his chapter "On Comparing Stories" he has this to say about how material for comparisons is chosen.
It is, to put matters bluntly, poor method to compare and contrast a richly nuanced and historically complex understanding of Pauline christology with a conglomerate of 'mystery texts' treated as if they were historically and ideologically simple and interchangeable; to treat the former as development and the latter as frozen. As long as we identify recognizable humanity with historical consciousness and openness to change with critical thought - as we do - the usual treatment o the religions of Late Antiquity, as well as the bulk of the other mythic traditions of humankind, is inhumane.
In other words, we can not, in our quest to be scholars and not mere "cheerleaders," as Bruce Lincoln would say, we must make sure that we are doing our sources justice. If we start out to compare "Christianity" with other religions, for example, in an attempt to show how "Christianity" is "unique," then our end necessitates that our means will be unjust to that which is not "Christianity" and even to that which we label as "Christianity," because we are, at best subconsciously, projecting our own normative view of "Christianity" onto the vast landscape of Christianities and declaring as "essential" or "true" Christianity, that which happens to be the form of "Christianity" that we happen to subscribe to today in light of our historical, social, economic, and political circumstances.