Grayling being a notable anti-theist, it is no surprise that he treats Christian views of friendship as an opportunity to take a few pot-shots at some large fish in a particularly small barrel. By doing so, he misses the chance to comment on a radical difference. In Cicero, for example, there is a vexed discussion of whether or not it is possible to be a true friend to someone who holds different political or ethical beliefs. The idea of treating people as if they were friends already seems to me to be a more profound shift in the concept than Grayling admits. He may have some fun with the idea that the infinite, self-sufficient deity should require being chums with sinners, but it is at the expense of realising that in religious ethics there is the very openness that he wishes for in terms of contemporary secular friendship. He praises the notion that “children in kindergarten will be unconsciously friends with anyone at all, of any persuasion, background, colour, faith or political family”. That one might consciously choose to befriend despite difference seems to me to be a religious rather than a philosophical proposition. The “as if” (treating people as if they were friends) is a leap of faith, not a cold piece of ratiocination.
Read my own thoughts on Christianity and friendship here.