The text reads, including the first verse, "And it came to pass, when men began to multiply on the face of the earth, and daughters were born unto them, that the sons of God saw the daughters of men that they were fair; and they took them wives of all which they chose."
Who the sons of God are may be learned from the 4th chapter, the last clause: "Then began men to call upon the name of Jehovah." Or, as the margin reads, "Then began men to call themselves by the name of Jehovah;" that is, a true, faithful people who believed in God and eschewed evil were called "the sons of God." Others, who did not, simply called themselves by the names which men gave them. And the daughters of men were the daughters of the unbelievers; and the sons of God made the sad mistake of taking to themselves unbelieving wives, and consequently the standard of religion was lowered, and the earth became corrupt. That is the simple explanation of it.
