Genesis 11:6

And the LORD said, Behold, the people is one, and they have all one language; and this they begin to do: and now nothing will be restrained from them, which they have imagined to do.

The people is one, and they have all one language. The multitudes assembled in the plains of Shinar formed an organized society, and began to establish the foundations of one universal empire. Union in councils gave them power, which was still further augmented by their ability to communicate their sentiments and designs easily and freely in a language universally intelligible; so that in such circumstances a confederacy of bold bad men might undertake the most daring enterprises.

This they begin to do - i:e., the building of the city and tower is but the commencement of their doings.

And now nothing will be restrained from them - i:e., they will shrink from nothing, however hard or presumptuous, which they may wish to accomplish; so that the evil already in the world will be fearfully increased, and its diffusion accelerated, by this ungodly association, unless means are taken for its immediate dissolution.