What was the sin of the tower of Babel?
The Lord came down to see the tower of Babel and said "If as one people speaking the same language they have begun to do this, then nothing they plan to do will be impossible for them. Come, let us go down and confuse their language so they will not understand each other." The Lord scattered them all over the earth. What was the big sin?
A Catholic Commentary on Sacred Scripture explains Genesis 11:1-9 as follows:
The inhabitants of the city started to build a great stage-tower zigguarat, which they raised to some considerable height and then were unable to complete. The massive remains were a visible reminder of the ineffectual attempt which tradition recorded to have been accompanied by discord and consequent migration of many of the population. This meager historical record explains why the sacred writer did not mention the nature of the sin which led to the frustration of the grandiose design. But his deep conviction of God’s overruling government of the world taught him that the attempt had been displeasing to God and the narrative suggests that the sin was one of overweening human pride and self-sufficiency.
The story has been widely understood to tell of a miraculous intervention by which different languages were introduced and the population thus became unable to understand one another. But it must be noted that the different groups among not only the Japhethites and Chamites, 10:5,20, but also among the Semites, 10:31, are already recorded to have had their various languages, and as the two former groups are already out of the story, the origin of languages is not recorded here. Moreover, the word for "languages" is lason which is that used in 10:5, 20, 31. In our passage this word is not found. That used is sapah, "lip," which also signifies "utterance." The natural meaning here is that the people at first in complete harmony "all saying the same thing" fell out among themselves and could not agree upon a common policy; and as a result of the discord (and possibly of fighting) there was a migration. Already in the fourth century St. Gregory of Nyssa was quite emphatic that God did not miraculously impose different languages on mankind. (192-193)
The Jerome Commentary on Genesis 11:1-9 says:
The evil is in their desire to "make a name" for themselves (cf. 12:2) rather than in the attempt to build a tower "with its top in the heavens" . . . Human smallness, not divine impotence, is emphasized in the Lord’s descent (vs. 5). The plural in verse 7 may reflect the concept of God’s royal court, an early idea in Israel. The punishment had been anticipated in verse 4. The name of the city, Babel, is here associated with the Hebrew root bll, "to confound." The great city and its (implied) defeat thus becomes synonymous with man’s revolt against God and its consequences. (17)