Some people wonder how God can be both loving and wrathful at the same time.   This verse is the second of four historical reminders starting with 'if' ending with the conclusion - that God knows how to punish the rebellious and save the repentant (2 Peter 2:9-10).  Today's illustration from Peter's catalogue of examples goes back to Noah and the flood.  You can read the story in Genesis 6:9-9:19.  God saw that everybody, apart from Noah's family, had been corrupted to the point of violence, and that there was no hope for them except to be destroyed (Genesis 6:11-13).  Not only would the people perish, but all the livestock as well.  But one family and an ark-full of animals survived to start again with God.
 
However, to focus on the attractive story of animals in the ark, is to miss the bigger message.  The great flood became an important object lesson to God's people of judgement and salvation (Genesis 6:5-8).  God did not hesitate to pronounce judgement on the wicked; however, He waited until Noah had built the ark.  During those long years of construction, God gave both time and explanation of His intentions to those who would later perish, through Noah who is described as a 'preacher of righteousness'.  

But Noah's faith did not excite others to join him; far from it.  Far from dismissing the Biblical flood narrative, as some might today, Jesus clearly taught the historical fact that Noah’s country people rejected the message, carrying on with their self-indulgent life until the torrential waters swept them away (Matthew 24:37-39) - proving that God's judgement was fair and just.  Yet, at the same time, God provided a way of salvation for Noah's family, who benefited because they believed.
 
All this was to help us understand Jesus.  He was God's ultimate way of salvation, of which the 'ark' is just a picture.  In Christ alone, the church will be safe in the final judgement, although that will be by fire and not by water (2 Peter 3:5-7).  Even Noah was not perfect after the flood (Genesis 9:20-29).  He and we all need the sacrifice of Jesus to atone for our sins (see www.crosscheck.org.uk to find out more).  You may say, if Noah preached righteousness and nobody listened, then he was not a good preacher!  But the message was right, even though the hearers did not listen.  It is the same today.  Although many may despise our godly lifestyle and gospel explanations, their refusal to listen to God's Word proves the rightness of their final judgement (Hebrews 11:7).  That is why we should keep witnessing to our friends and colleagues while there is still time for them to repent.