This is an interesting thought, but how could we get everyone to join us to unite and abandon religion all at once. I guess we can just wait and join together as people leave the fold but as long as there is religion there will always be separation and the world will never be united as one.

There is only are only two ways we can all be united and that is that we all agree on one belief system which entails no separation, every last one of us or we all let go of belief or faith all together but then even still I think, we would find ways to separate ourselves as I fear it is human nature.

I don't think there is a way to unite humanity unless we can find a threat large enough and non-human enough to force us all to work together. But even still in the presence of that religion would hold us back from ever being fully united.

If a god were real and he really wanted this though, he would have two options aliens that would somehow make everyone question their faith or making his, her or it's presence known.