Robert Candlish from Studies in Genesis (commenting on Gen. 6:1-22):

When the Lord first fixed the period of his long-suffering patience, and resolved to spare man on the earth for one hundred and twenty years, and no longer—he doubtless intimated this purpose in some way; announcing the destruction coming on all flesh, and giving some public pledge of the grace which Noah found in his eyes. This would be a new call to Noah to labour in his vocation of a teacher of righteousness, as well as a loud alarm to the world at large. Noah obeyed the call;—the world set at naught the alarm. To Noah it was indeed a trying office that was assigned—to testify for God in the midst of such a generation, to whose hatred and jealousy the very token of approbation with which God had honoured him, tended the more to expose him. He became a marked man, the object of scorn and contumely, of injury and insult. The people watched for his halting, and waylaid his path with subtle snares. It was a difficult part he had in these circumstances to perform—a dangerous duty he had to discharge—as he walked humbly with his God and went about warning the ungodly.