The Neanderthals are an interesting species though unfairly treated. They are frequently portrayed as knuckle-dragging idiots, serving as a metaphor for people with retrograde views, especially with respect to gender and sexuality, though there is no reason to think that they were any worse than the ancestors of our own species at that time.
It used to be thought that they may have been part of our direct ancestral line, but the discovery a few decades ago of some early human fossils showed that Neanderthals existed contemporaneously with our own ancestors and so they are on a different branch of the evolutionary tree. They are our cousins but thought to be a distinct species, just like gorillas.
But things changed in 2010. Ann Gibbons writing in Slate says:
All that changed in May 2010, when researchers were able to get enough nuclear DNA from three female Neanderthals who lived in a cave in Croatia 38,000 to 44,000 years ago to splice together and publish the first draft of a Neanderthal genome. When they compared that draft genome with DNA from modern humans in Europe, Asia, and Africa, paleogeneticist Svante Pääbo of the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, and his colleagues found that modern Europeans and Asians—but not Africans—have inherited between 1 percent and 4 percent of their genes from Neanderthals. They proposed that we inherited this DNA from a few close encounters between our ancestors and Neanderthals, perhaps after modern humans swept out of Africa and into the Middle East but before they spread into Europe and the rest of Asia. This was not wholesale intermingling—nor was it classic multiregional continuity—but low-level interbreeding, they said.
But that’s not all. Researchers also found fossils of another type of archaic human in the Siberian region whom they have called the Denisovans and have been able to extract their DNA. They have identified people living now who have about 3% of their DNA corresponding to Denisovans and 4% to 6% with Neanderthals.
These kinds of discoveries tend to pose problems for Christians. One of the key elements of Christianity, even for those who accept evolution in general, is that there is something special about human beings. The idea that our fairly recent ancestors were close enough to Neanderthals and Denisovans to interbreed with them disturbs the idea that there is something unique about us that makes us special in the eyes of god, worthy of salvation.
It would not help to include the other two species as also being human since you then have to explain why god would choose to make extinct his special creations.