8 Even this then was truly loving-kindness in God, but little as yet in comparison with what follows. For consider what happened in the days of Noe. The giants sinned, and much wickedness was then spread over the earth, and because of this the flood was to come upon them: and in the five hundredth year God utters His threatening; but in the six hundredth He brought the flood upon the earth. Do you see the breadth of God's loving-kindness extending to a hundred years? Could He not have done immediately what He did then after the hundred years? But He extended (the time) on purpose, granting a respite for repentance. Do you see God's goodness? And if the men of that time had repented, they would not have missed the loving-kindness of God.
Source: Catechetical Lectures (New Advent)