Seriously. Harvesting souls through the behind? There are many other open cavities that a person has. And where does this divine revelation that a body actually houses the soul come from, and that it can be stolen away by an alien with a probing device, considering that the soul has a spiritual existence, not a existence in reality?
Apart from that, the divine revelation that a soul can be stolen leaves us with the opportunity to actually prove the existence of a soul, because there will be those with a soul, and those without.
If they had their own breeding colony, all of their race could join in the soul-harvesting fun.
And considering that you have not made a claim to how much these aliens are spread, six billion odd people might not even be close enough to get enough souls for them. Perhaps they have a race of hundreds of billions of beings.
This sounds an awful lot like scientology. Can anyone support this, because I have little experience in that religion.
How come many of them formed their own Churches, then? Martin Luther was inspired to create the Protestant religion following the excesses and hypocracy of the Catholic Church. Joseph Smith created Mormonism. More recently, L Ron Hubbard created the Church of Scientology.