Study uncovers lost 'golden age' of languages

That's not even Biblical, as God provides Cain with protection via the mark that he placed on Cain. The building of the city has, as I pointed out, nothing to do with self-protection from "loss of God's grace" (nowhere in the Bible does it say that Cain lost God's grace, and the mark itself as in act of grace to save Cain from being killed).

Let's look at standard Christian exegesis:

* Augustine provides the definitive patristic interpretation. According to Augustine, Cain building a city was an expression of his desire to remain tied to the land (a tiller of the soil) rather than a wanderer (cattle herding, as Abel). See The City of God (Book XV, Chapters 1 and 20). Augustine picks up on this as it is his own theme of the two cities: the earthly city (civitas terrena) and the heavenly city (civitas dei). Augustine says that Cain, as the first murderer, founded the earthly city, whereas Abel was a pilgrim and stranger who founded nothing on earth, but entered into the heavenly city.

* Later medieval commentators, such as Nicholas of Lyra and the Glossators, reinforced Augustine's interpretation, contrasting Cain's urbanization with the nomadic, tent-dwelling lifestyle of the patriarchs, accusing Cain of being worldly in his desires. E.g. it was an expression of Cain's desire, his teleos, and not a defense mechanism.

* Moving onto the reformation, in Lectures on Genesis, Luther views Cain as the archetype of the secular, worldly church opposed to the suffering church of Abel. Luther notes that while those who look for the heavenly city choose to dwell in tents as pilgrims, Cain builds a permanent habitation. Constructing a city represents an attempt to find rest, satisfaction, and permanence in the earthly sphere precisely because he is cut off from God.

* Looking at John Calvin, in Commentary on Genesis he also agrees with Augustine, arguing that Cain's establishment of a city marks the entrenchment of a society organized around immanent, earthly flourishing rather than faith.

Interestingly, some rabbinical commentators argue that in Lamech's grisly song "I have killed a man", that Lamech is talking about Cain -- e.g that Cain was murdered, accidentally, by Lamech, his descendant. The midrash in Genesis Rabbah is that the mark of Cain was a horn that grew out of Cain's head, and Lamech was a blind hunter who was guided by his son. The son saw a movement in the bushes and the horn, mistaking it for a wild animal, and Lamech fired his arrow, killing Cain. Thus God's grace extended only to seven generations. But Christian commentators do not argue for a seven generation time limit on God's protection, and assume Cain died of natural causes.

Exactly, and yet he still built a walled city. Why? For the same reason why Adam and Eve suddenly realized they were naked after falling from grace.

P.S. Neither Roman Catholics nor Protestants have anything approaching a standard Christian exegesis. Both traditions are way out there and are diverging significantly from Christianity.