The situation is not just theoretical. There are, in fact, nations of entirely mentally handicapped people.
Anyone with an IQ below 70 is officially regarded as mentally retarded. 
Around the world there are several populations of pygmy people. These pygmies are considered to be racially distinct from the populations that surround them. They exist in India, Africa, and Asia. Typically, these people look like miniature black Africans.
Tests reveal that the average member of these pygmy populations has an IQ of only 53.  Such low scores indicate that very few, if any, members of these ethnic groups operate above the level of cognitive retardation.
This situation creates a problem for Christians. What should be done with these populations? Should they be regarded as equal Christian brothers and sisters, or should they be treated like children?
An IQ of 53 is lower than some Chimpanzees, and closer to that of intelligent animals than to modern Eurasian human populations. 
Disparities in intelligence between ethnic groups (i.e. "nations") should be addressed by Christian thinkers.
Modern Christianity, swept up in the "equality" fad, has tended to ignore big questions about genetic differences between populations, and how these differences may contribute to our interpretation of the human landscape.
 Euroethnic and East Asian populations have an average intelligence near IQ 100. Chimpanzees, Dolphins, Crows, and Parrots sometimes have IQs as high as 40 or 50. Pygmy populations are closer to these species' intelligence than that of Eurasians.
Coojimans, Paul. "I.Q. and real-life functioning." Accessed March, 2016. http://paulcooijmans.com/intelligence/iq_ranges.html.
EXCERPT: "The most intelligent non-human animals, such as some crows, chimpanzees, bonobos, parrots, and dolphins, are in this range [IQ 35 to 49]. Bonobo or chimpanzee I.Q. scores are sometimes even quoted as high as 80 or 90, but those are childhood age-peer scores that correspond to adult I.Q.'s of only just over 40."
 Lynn, Richard. "Intelligence of the Pygmies." Mankind Quarterly. Summer, 2011. Accessed March, 2016. https://www.questia.com/library/journal/1P3-2380310781/intelligence-of-the-pygmies.
EXCERPT: "The intelligence of the Biaka and Babinga Pygmies and of Negroids living in the same region of the Central African Republic is examined with four tests of cognitive abilities. The Pygmies scored lower than the Negroids on all tests by an average of 14 IQ points. In relation to a European IQ of 100, the IQ of the Pygmies is estimated at approximately 53."