The argument for: The fact that someone wanted the land they were on. They were making very good use to it prior to their removal. But some whites wanted land because they couldn't make a profit from it.
First, they argued that the white Americans (and their black slaves) needed the land and could make better use of it. They argued that Americans could make the land more economically productive than the Indians had been able to. Second, the proponents argued from the idea of white superiority. They argued that the Indians were inferior and that it was not necessary to treat them fairly.
Because there was a lot of Apostles that spread the word and traveled around the world. Also, Jesus was alive and doing a lot miracles and made pople believe.