Man's sin is the cause of the deluge. Noe is commanded to build the ark.

6:1. And after that men began to be multiplied upon the earth, and
daughters were born to them,

6:2. The sons of God seeing the daughters of men, that they were fair,
took to themselves wives of all which they chose.

The sons of God... The descendants of Seth and Enos are here called sons
of God from their religion and piety: whereas the ungodly race of Cain,
who by their carnal affections lay grovelling upon the earth, are called
the children of men. The unhappy consequence of the former marrying with
the latter, ought to be a warning to Christians to be very circumspect
in their marriages; and not to suffer themselves to be determined in
their choice by their carnal passion, to the prejudice of virtue or
religion.

6:3. And God said: My spirit shall not remain in man for ever, because
he is flesh, and his days shall be a hundred and twenty years.

His days shall be, etc... The meaning is, that man's days, which before
the flood were usually 900 years, should now be reduced to 120 years. Or
rather, that God would allow men this term of 120 years, for their
repentance and conversion, before he would send the deluge.

6:4. Now giants were upon the earth in those days. For after the sons of
God went in to the daughters of men, and they brought forth children,
these are the mighty men of old, men of renown.

Giants... It is likely the generality of men before the flood were of a