"They will say, 'Where is this "coming" he promised? Ever since our fathers died, everything goes on as it has since the beginning of creation.'" NIV translation
These scoffers and false teachers will basically say that Jesus is not coming back for His people. For those that know Jesus, His coming is the fulfillment of our hope. Without that hope, many will live for today and the false pleasures that this world offers. These false teachers will point to the past and say that He hasn't come yet so He is not coming. This, in turn, will cause many to live like there is no tomorrow and no coming judgment of sin.
"But they deliberately forget that long ago by God's word the heavens existed and the earth was formed out of water and by water. By these waters also the world of that time was deluged and destroyed. By the same word the present heavens and earth are reserved for fire, being kept for the day of judgment and destruction of ungodly men." NIV translation
It is not that these false prophets do not know the truth but they simply deny it. The truth is that God created everything by His Word and He passed judgment on the world in Noah's day through the flood.
It is also true that God has said that the present earth is going to be judged with fire. These so-called prophets know it is coming but choose to ignore it and there are more of these "prophets" today than ever before.
"But do not forget this one thing, dear friends: With the Lord a day is like a thousand years, and a thousand years are like a day." NIV translation
We often forget that God created time (in Genesis) and He is in control of time. We think of time in terms of our lifespan but God is eternal and what seems like a long time for us is only a speck of time to Him. Those who say it has been so long since Jesus said He was coming back, forget that it is all in relation to eternity.
"The Lord is not slow in keeping his promise, as some understand slowness. He is patient with you, not wanting anyone to perish, but everyone to come to repentance." NIV translation
In our world today with such an obsession with sports and athletic ability, we place a premium on speed. In doing this, many confuse God's patience in delaying the judgment with weakness. As Peter tells us, it is not out of weakness that the judgment is delayed but out of love and the fact that God does not want to see any human being cast into hell.