This looks like poor writing. I interpret "humanization" as something like "recognition of the other" as in, you know, "seeing people as people" rather than as "obstacles to my own enrichment" or "threats" or something else. Dehumanization is the opposite. That part is me guessing, but most of the rest of this I am confident about. My rephrasing, with "humanization" as "X" because I'm just guessing at the meaning:

A person concerned for X immediately notices that anti-X is not only something that can theoretically happen, but something that has been happening all through history up to this day. Because of the extent of anti-X, [the aforementioned person] may feel the task of replacing anti-X with X to be impossible. Despite this discouragement, it is still the case that either path is possible for people as changing beings.

I'm also just kind of guessing at what he means by "incompleteness".