Doctrine of Discovery

The Doctrine of Discovery established a religious, political, and legal justification for colonization and seizure of land not inhabited by Christians.  Those who "did not know Christ" were are regarded as heathens who are incapable of owing or managing land.  Terra nullius is an aspect of the Doctrine of Discovery.  So even if there are native inhabitants, if they are not using the in the way that Europeans approved, ie farming productively, then that land is considered vacant and available for colonisation.

Foundational elements of the Doctrine of Discovery can be found in a series of papal bulls, or decrees, beginning in the 1100s, which included sanctions, enforcements, authorizations, explusions, admonishments, excommunications, denunciations, and expressions of territorial sovereignty for Christian monarchs supported by the Catholic Church.

Queen Elizabeth I said to Sir Walter Raleigh and Sir Humphery Gilbert "Go where no Christian prince has yet been and acquire for me sovereignty, jurisdiction and title.