If your moral authority is a god, then when a person says "God wants you to take this land and make it his" you might question the person, but either way you must struggle with the idea that perhaps your god does want you to take a native people and move them (and kill them a little bit). No matter what, you can't appeal directly to your authority.
If your moral authority is human, then it can be challenged. It's decree to take land would have to be defended and rationalized. You can resist or comply, but the ambiguity that is present in religious thought is gone.
If a Christian man tells a Christian lawyer that he killed a person because God told him to do so, how can the lawyer morally resolve this? Can he say that the man is lying? I mean what if the man actually did talk to God and was told to kill? His action would be moral then right? On the other hand, he is a representative of a human moral authority. An authority that says the action is morally wrong; an authority that will defend this position. No god has ever shown up to court, no matter how many times it's been asked to come and defend actions it is claimed to be party to.
The idea of a god, and that a god command and judges humans, combine into a means to exploit and manipulate people. It is a mental vulnerability.
Watching US politics should be tell tale of this. Nowhere else will you see a nationalistic idea of religious entitlement sewn so tightly with moral condemnation of the "godless."