If I cannot tell which religion is true (do Catholics go to Heaven or Hell? What about Mormons? Baptists?), how can I be responsible? For a law to be just and fair, it must be able for it to be known to be true. For example, I can look at every act of parliament and every statute and regulation. I can see the courts in operation and know that these laws are the ones I should obey, and not the laws of Afghanistan, for example.
It is impossible for me to tell which laws I should be following and what God. Thus, God is manifestly unfair and unjust if he condemns me to eternal punishment. (The reason I bold this is that sometimes I get the feeling that Christians do not understand exactly what this means).