We might tell him that he could share with us and join us in our work
The text from the start of the paragraph to this point asks what we would do if someone came out of the blue and claimed everything we had as his, we would not heed him any attention but would go on about our lives.
But if he should have a government back of him, he would appeal to it for the protection of "his rights," and the government would send police and soldiers who would evict us and put the "lawful owner in possession."
A man who claims to have backing from an authority(government) can use the law to loop around and take from people what is rightfully theirs and not his.