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

¹ And when men began to be multiplied upon the earth, and daughters were born to them,

² the sons of God, seeing that the daughters of men were beautiful, took to themselves wives from all whom they chose.¹²

1. 6:2Augustine and others say that this passage refers to the lineages of Seth and Cain, the one being holy and close to God, and the other living apart from God. My suggestion is a more generalized version of what Augustine said, namely, that the sons of God are those who stayed close to God in their lives, and the sons of men are those who lived merely secular lives. In the Bible, the term ‘sons of’ does not necessarily connotate lineage. For example, those who commit adultery are called, ‘sons of adulterers,’ not because their parents were also adulterers, but because they act as if they were sons of adultery personified. There were, in ancient times, prior to any organized religion as we have today, some who were holy, who lived close to God, and who, as a result, lived long lives. But these sons of God were attracted to women who were not holy, who lived secular lives. Thus God generally limits the length of our lives because of sin.(Conte)
2. 6:2The 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.(Challoner)

³ And God said: “My spirit shall not remain in man forever, because he is flesh. And so his days shall be one hundred and twenty years.”¹

1. 6:3His 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.(Challoner)

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 conceived, these became the powerful ones of ancient times, men of renown.¹²

1. 6:4The giants were perhaps those who were giants in their mental stature, not so much in their physical stature. There may also have been stark differences in the physical characteristics of different groups of people, due to small gene pools, in groups separated by tribe or lineage. Some groups may have been markedly larger or stronger than others. The sons of the sons of God and the daughters of men were mighty because God blesses the offspring of those who are holy.(Conte)
2. 6:4Giants: It is likely the generality of men before the flood were of a gigantic stature in comparison with what men now are. But these here spoken of are called giants, as being not only tall in stature, but violent and savage in their dispositions, and mere monsters of cruelty and lust.(Challoner)

⁵ Then God, seeing that the wickedness of men was great upon the earth and that every thought of their heart was intent upon evil at all times,

⁶ repented that he had made man on the earth. And being touched inwardly with a sorrow of heart,¹

1. 6:6It repented him, etc: God, who is unchangeable, is not capable of repentance, grief, or any other passion. But these expressions are used to declare the enormity of the sins of men, which was so provoking as to determine their Creator to destroy these his creatures, whom before he had so much favoured.(Challoner)

⁷ he said, “I will eliminate man, whom I have created, from the face of the earth, from man to other living things, from animals even to the flying things of the air. For it grieves me that I have made them.”

⁸ Yet truly, Noah found grace before the Lord.

⁹ These are the generations of Noah. Noah was a just man, and yet he was predominate among his generations, for he walked with God.¹

1. 6:9Even though Noah lived among in evil generation, he was well-thought of and successful among them, because he walked with God. This verse also has the sense that Noah was predominate among his own family and descendents because he walked with God. And it also means that Noah was uncorrupted by his time.(Conte)

¹⁰ And he conceived three sons: Shem, Ham, and Japheth.

¹¹ Yet the earth was corrupted before the eyes of God, and it was filled with iniquity.

¹² And when God had seen that the earth had been corrupted, (indeed, all flesh had corrupted its way upon the earth)

¹³ he said to Noah: “The end of all flesh has arrived in my sight. The earth has been filled with iniquity by their presence, and I will destroy them, along with the earth.

¹⁴ Make yourself an ark from smoothed wood. You shall make little dwelling places in the ark, and you shall smear pitch on the interior and exterior.

¹⁵ And thus shall you make it: The length of the ark shall be three hundred cubits, its width fifty cubits, and its height thirty cubits.¹

1. 6:15Three hundred cubits, etc: The ark, according to the dimensions here set down, contained four hundred and fifty thousand square cubits; which was more than enough to contain all the kinds of living creatures, with all necessary provisions: even supposing the cubits here spoken of to have been only a foot and a half each, which was the least kind of cubits.(Challoner)

¹⁶ You shall make a window in the ark, and you shall complete it within a cubit of the top. Then you shall set the door of the ark at its side. You shall make in it: a lower part, upper rooms, and a third level.¹

1. 6:16So, the top of the window was placed so that it would be within a cubit of the summit of the arc. Then Noah could see a greater distance from the window than if it were placed lower, and it would be less susceptible to leaks from waves hitting the window.(Conte)

¹⁷ Behold, I shall bring the waters of a great flood upon the earth, so as to put to death all flesh in which there is the breath of life under heaven. All things that are on the earth shall be consumed.

¹⁸ And I shall establish my covenant with you, and you shall enter the ark, you and your sons, your wife and the wives of your sons with you.

¹⁹ And from every living thing of all that is flesh, you shall lead pairs into the ark, so that they may survive with you: from the male sex and the female,¹

1. 6:19This verse does not yet say how many pairs, but merely instructs that every living creature placed on the ark should be in pair, male and female. This passage teaches us that human persons, as well as all other animals, are intended by God to be in pairs.(Conte)

²⁰ from birds, according to their kind, and from beasts, in their kind, and from among all animals on earth, according to their kind; pairs from each shall enter with you, so that they may be able to live.

²¹ Therefore, you shall take with you from all the foods that are able to be eaten, and you shall carry these with you. And these shall be used as food, some for you, and the rest for them.”

²² And so Noah did all things just as God had instructed him.