There can be no such onlooker. If the whole truth is available to anybody, then that person could tell the blind men, and they wouldn't disagree.
2. Christianity (biblical monotheism) holds that there is much about God which is unknown so there will be blind spots.
Which is a direct contradiction of your point 1 (above)
3. His own Christianity holds that there is only one true elephant which, since there is no other, must therefore be the one in the story.
And yet other religions have devotees with equal faith and certainty to that of Christians, which is the very issue this story is supposed to address.
The most important point about the story behind the image is atheism versus theism.
All of the men, despite their degree of blindness, agree that something exists.
But the worst form of blindness is that of the atheist who does not feel anything yet tells the blind men they are ALL wrong.
Not at all.
The big problem for theists is "If my religion is the correct one, how can there be people with equal faith who believe in other, incompatible religions?"
This is impossible, as the correct religion, supported by God, would obviously completely destroy any false religions. But as none does, there must be something else going on.
The idea that all religions are different experiences of the same fundamental truth is a nice excuse for the fact that reality contradicts faith; but it only works if you avoid thinking too hard about it.
There cannot be an elephant. The core beliefs of different religions are mutually exclusive. Thor and Odin are not just a different way of looking at Jesus and Jehova. They are utterly different things, and both cannot be simultaneously true.
Christians are as certain that the Jewish, Muslim, Hindu, Sikh and Shinto core beliefs are false as they are that atheism is false. If they were not, then Christianity would look radically different.
Christianity isn't the belief that some kind of mysterious supernatural 'thing', that could possibly be the Hindu pantheon, exists. Christianity requires belief in the divinity of Jesus; if that bit is missing from your beliefs, then whatever you are, you ain't a Christian. But Islam requires belief in Jesus as a mere prophet - the very possibility of his divinity contradicts the core message of Allah's unity and indivisibility. These are not views of different parts of the same larger mysterious elephant; they are incompatible positions regarding the exact same character.
The existence of the elephant is not only unknowable; it is demonstrably not the case. The atheist position that the elephant doesn't exist is the only one with evidentiary support.
