The ultimate criterion by which we must judge the value of a religion is not its cultural fruits but its spiritual truth. This, however, is not the criterion which the historian or the sociologist applies in his judgment of an age or a civilization. A false religion which produces a great art or a great literature, a religion which expresses itself in a brilliant civilization will naturally be of greater interest to him than a true religion which produces only martyrs or mystics. But while the historian is justified in judging the cultural value of a religion by its cultural fruits, he has no right to treat his conclusions as final from the religious point of view. Actually, however, it is very difficult for an historian to preserve this distinction between religious and cultural values. If he believes a religion to be true, he will naturally tend to take a favourable view of the culture with which it is associated, and if he regards a culture as barbarous or unprogressive he will be apt to condemn or depreciate its religious standards and beliefs.
And then, a description that could just as well be put to the recent column and comments of Professor Grayling:
Now it was on this ground that the traditional humanistic criticism of medieval religion was based. Medieval literature, medieval philosophy and medieval science alike appeared beneath contempt in the eyes of the Renaissance scholar, and still more of the philosopher of the eighteenth-century Enlightenment, and consequently medieval religion either shared in their condemnation or, still more frequently, was regarded as primarily responsible for the cultural backwardness of medieval Europe--in Gibbon's famous phrase, the Middle Ages were "the triumph of barbarism and religion". 
But, Dawson notes--in 1934--that such views were no longer tenable, nor in vogue:
This wholesale condemnation of medieval culture has long since been abandoned by the educated world, and it was the rediscovery of the purely cultural values of the Middle Ages--of medieval literature and medieval art--which was the main factor in bringing about the change, and which contributed very materially to the wider appreciation of the value of medieval religion. 
And yet Grayling and others are able to be so unremittingly negative about the history of Christianity in general and the medieval era in particular because there remains, for various reasons, a huge chasm between scholarly research and popular knowledge. As Grayling's column indicates (and as he even tacitly admits), appealing to popular prejudices and longstanding stereotypes about the "dark ages" is often a successful polemical tactic.
Read parts one and two of Olson's response