First, Paganism halts the progress of GOD. Pagan religions such as the Aspectist belief of the Wood Elves and the Dominion corrupts the minds of faithful by confusing them and taking them away from the light of GOD. These religions have little morals and gives a chance for those with corrupt minds to show themselves within socialized societies. With immoral religions, come immoral and dangerous people for all sentient beings. Unlike those of the true faith, pagans have little cognitive ability.
Additionally, Pagans such as Aspectists corrupt the mind, forcing them into an isolated state of being. They can't listen to reason as false Gods flood their minds with anti-Canonist beliefs. They get stuck in a corrupt plane of existence that could have only one way out; death. The only way to free the soul from the grasps of such dark and sick minds is by putting the poor being to finally rest. These religions make the person hateful, and distant. It can be seen by studying subjects of the Dominion. The Aspects have made them hateful, ignorant, and angry. The Dark Elves of the Dominion have always had it rough for praising their ancestors and forgetting about the Creator of all beings; the creator of their own ancestors.
Finally, the most important reason as to why paganism halts the progress of GOD. Pagan religions corrupt the minds of the faithful by confusing them and taking them away from the light of GOD. Pagan worship splits up those of weak faith, turning them into heathens along the line. With more and more pagan believers, more people will be turned. First it grows slowly; but then it spreads like a sickness. It is terrible watching those of pure nature turn to craven arts against the Church.
Indeed, some would say that these religions are not harmful. In reality, they only bring those with different views from the Church to a false faith; but any belief that is not a canonist belief is an abomination in the eyes of GOD. In cases that conversion to the true faith has been denied, freeing the soul from the grasps of the body is the only true solution.