Juan Luis Segundo completed his theological studies at Louvain and received his Doctorate of Letters from the Sorbonne. He was chaplain to various groups in his native Uruguay. He taught theology at the Universities of Harvard, Chicago, Toronto, Montreal, Birmingham, and Sao Paulo.
I do not have a background in Catholic theology so not all of the ideas were relevant to me, in addition to which a lot of the language seemed unnecessarily formal which may have to do with this text being 46 years old and in translation.
Segundo, a priest, identifies the traditional Catholic concept of sin and wants also to embrace the scientific concept of evolution, particularly as it applies to moral development on an individual and cultural level. His imagined ideal representative of moral theology would say that “man, by his very nature, is permanently confronted with a choice between opposites. He must choose between a which is good and b which is evil." (p. 8) A man has (or can obtain) knowledge of the correct choice and he has freedom to act upon it, albeit with somewhat less awareness and power than he would have originally had in the Garden of Eden which can never be recovered. No matter how the man might grow personally, the moral choice placed before him does not change, and so Segundo refers to this entire religious position as "immobilist." But Segundo also endorses what his imagined scientist would say: The behavior we consider to be moral evolves according to whatever behaviors meet survival needs, and any individual's knowledge of the moral rule, as well as his free will to act upon it, is limited by genes and environment. Humanity itself is changing: "Man is not a finished, readymade concept," and "what we call moral conscience is still in gestation even today." (pp. 9-10) To "'relativize' in the good sense of the word" is to ask "For whom is it good?" (p. 17)
It is Segundo's project in this book to reconcile these two positions. He does this in part by saying that the “dialectic between habit and conservatism on the one hand and innovation and crisis on the other hand” (p. 59) — obvious though it may seem to some — is the key to Christianity. The “critical questioning” of both of sides creates the dialectic.