The relativist looks at the subject of religion, and says:
"A man's religion depends on his upbringing - a Christian would not be a Christian but for it, and the same goes for the Muslim, Jew, or Hindu. Had the Christian been brought up in a Muslim culture he would likely be a Muslim; had the Muslim been brought up in a Hindu culture he would likely be a Hindu. There is no reason to accept religion except for the cultural and social pressures to do so... there is no God, or at least none that we know of, "God" is just a concept we have created which exists solely in our minds."
Now, so far, it seems very likely that our relativist is talking sense - we may be inclined to agree with him, and we find that his explanation of religion is certainly simpler and therefore more initially plausible than that of the believer. After all, if there is a god, how would we know anyway? And would this be the Christian's god, the Muslim's, the Hindu's, or some other?
But now, if we accept the analogy of religious relativism with moral relativism, we get the most absurd conclusion. The relativist - the same relativist - might continue:
"I was brought up in a Christian culture, therefore I accept the Christian standards: I go to church on Sundays, I pray to the Lord before meal times, I give my tithe to the church, and I would never break the Ten Commandments or do anything contrary to the guidance contained in the Bible."
The relativist would have to admit the above, provided he had been raised in a Christian culture, but, had he been raised as a Muslim or Hindu, he would have to make similar corresponding statements.
The question immediately presents itself - if the relativist does not accept that his religion is true, what grounds does he have for keeping it (as a guide to action)? Surely the fact that there are no grounds, except for a culturally-induced deception, would sensibly be taken as grounds for being (and acting as the) agnostic, or perhaps atheist, not a supporter of the locally-popular religion (whichever that happened to be)? And the same question must be asked of the moral relativist - if a moral law or standard is not true, or objectively valid, why on earth would someone (knowing this) accept it as suitable guidance by which to live their life? And religion is analogous to morality in the relevant respects - it is no matter that religious behaviour is generally inspired by a belief that the relativist does not share, for the same is also true of morality: common sense morality would have us believe that some acts (e.g. murder and rape) are objectively wrong, so if the relativist would refrain from murder and rape despite this refrain being usually based on a belief he does not share, he must also agree to behaviour inspired usually by a belief in god which he does not share.