This comes in here as a token of God's displeasure at those who married strange wives; he threatens to withdraw from them his Spirit, whom they had grieved by such marriages, contrary to their convictions: fleshly lusts are often punished with spiritual judgments, the sorest of all judgments. Or as another occasion of the great wickedness of the old world; the Spirit of the Lord, being provoked by their resistance of his motions, ceased to strive with them, and then all religion was soon lost among them. This he warns them of before, that they might not further vex his Holy Spirit, but by their prayers might stay him with them. Observe in this verse,
I. God's resolution not always to strive with man by his Spirit. The Spirit then strove by Noah's preaching (Pe1 3:19, Pe1 3:20) and by inward checks, but it was in vain with the most of men; therefore, says God, He shall not always strive. Note, 1. The blessed Spirit strives with sinners, by the convictions and admonitions of conscience, to turn them from sin to God. 2. If the Spirit be resisted, quenched, and striven against, though he strive long, he will not strive always, Hos 4:17. 3. Those are ripening apace for ruin whom the Spirit of grace has left off striving with.
II. The reason of this resolution: For that he also is flesh, that is, incurably corrupt, and carnal, and sensual, so that it is labour lost to strive with him. Can the Ethiopian change his skin? He also, that is, All, one as well as another, they have all sunk into the mire of flesh. Note, 1. It is the corrupt nature, and the inclination of the soul towards the flesh, that oppose the Spirit's strivings and render them ineffectual. 2. When a sinner has long adhered to that interest, and sided with the flesh against the Spirit, the Spirit justly withdraws his agency, and strives no more. None lose the Spirit's strivings but those that have first forfeited them.
III. A reprieve granted, notwithstanding: Yet his days shall be one hundred and twenty years; so long I will defer the judgment they deserve, and give them space to prevent it by their repentance and reformation. Justice said, Cut them down; but mercy interceded, Lord, let them alone this year also; and so far mercy prevailed, that a reprieve was obtained for six-score years. Note, The time of God's patience and forbearance towards provoking sinners is sometimes long, but always limited: reprieves are not pardons; though God bear a great while, he will not bear always.

My spirit shall not always strive - It is only by the influence of the Spirit of God that the carnal mind can be subdued and destroyed; but those who wilfully resist and grieve that Spirit must be ultimately left to the hardness and blindness of their own hearts, if they do not repent and turn to God. God delights in mercy, and therefore a gracious warning is given. Even at this time the earth was ripe for destruction; but God promised them one hundred and twenty years' respite: if they repented in that interim, well; if not, they should be destroyed by a flood. See the note on Gen 6:5

And the Lord said,.... Not to Noah, as in Gen 6:13 for, as yet, he is not taken notice of, or any discourse addressed to him; but rather to or within himself, he said what follows, or thus concluded, and resolved on in his own mind:

my Spirit shall not always strive with man; meaning either the soul of man, called the Spirit of God, Job 27:3 because of his creation, and is what he breathes and puts into men, and therefore is styled the Father of spirits; and which is in man, as some in Aben Ezra observe to be the sense the word used, as a sword in the scabbard; and so the meaning is, it shall not always abide there, but be unsheathed and drawn out; man shall not live always, since he is corrupt, and given to carnal lusts: or else, as Jarchi thinks, God himself is meant, and that the sense is, my Spirit shall not always contend within myself; or there shall not always be contention within me concerning man, whether I shall destroy him, or have mercy on him; I am at a point to punish him, since he is wholly carnal: or rather this is to be understood of the Holy Spirit of God, as the Targum of Jonathan, which agrees with Pe1 3:18 and to be thus interpreted; that the Spirit of God, which had been litigating and reasoning the point, as men do in a court of judicature, as the word signifies, with these men in the court, and at the bar of their own consciences, by one providence or by one minister or another, particularly by Noah, a preacher of righteousness, in vain, and to no purpose; therefore, he determines to proceed no longer in this way, but pass and execute the sentence of condemnation on them:

for that he also is flesh; not only carnal and corrupt, but sadly corrupted, and wholly given up to and immersed in sensual lusts and carnal pleasures, so as not to be restrained nor reformed; even the posterity of Seth, professors of religion also, as well as the profane world and posterity of Cain:

yet his days shall be an hundred and twenty years: meaning not the term of man's life, reduced to this from the length of time he lived before the flood; but this designs the space that God would give for repentance, before he proceeded to execute his vengeance on him; this is that "longsuffering of God" the apostle speaks of in the afore mentioned place, "that waited in the days of Noah, while the ark was preparing"; and so both the Targums of Onkelos and Jonathan interpret it of a space of an hundred and twenty years given them to repent: now whereas it was but an hundred years from the birth of Japheth to the flood, some think the space was shortened twenty years, because of their impenitence; but it is more probable what Jarchi observes, that this decree was made and given out twenty years before his birth, though here related, by a figure called "hysteron proteron", frequent in the Scriptures.

flesh--utterly, hopelessly debased.

And the Lord said, My spirit shall not always strive--Christ, as God, had by His Spirit inspiring Enoch, Noah, and perhaps other prophets (Pe1 3:20; Pe2 2:5; Jde 1:14), preached repentance to the antediluvians; but they were incorrigible.

yet his days shall be an hundred and twenty years--It is probable that the corruption of the world, which had now reached its height, had been long and gradually increasing, and this idea receives support from the long respite granted.