Peter then delivers the contrast of his examples. "For if God (did these)...The Lord knoweth how to deliver the godly out of temptations, and to reserve the unjust unto the day of judgment to be punished..." In the midst of the necessary destruction of the wicked, the Lord knows how to deliver his own from that destruction, while still reserving the unjust to the day of judgment. It's not an all-encompassing bill, or chemical warfare that gets the target as well as has collateral damage. God knows exactly who His targets are and moves with deadly accuracy. This verse makes perfect sense looking at Peter's examples. When a third of the angels sinned with Lucifer, God did not cast out all the angels-He cast out those who sinned. When the old world was full of violence and ungodliness, God saw righteous Noah and saved him and his family, and continued humanity through him. When Sodom and Gomorrah cried out for punishment, God saved His own backslidden child, Lot, and whoever of his family would come with him. Through these examples we know that the Lord will save Christians out of temptations.
The application: Temptations can have a three-fold meaning, and all are great promises. The first is literal temptation. When Satan tempts us, we can know that it is not more than we can bear and that God gives us a means of escape, always through His Word and sometime other ways as well (I Cor. 10:13). Second is all of the trials that we face hear on Earth. Psalm 34:15-22 says that "the eyes of the LORD are upon the righteous", and that even though their afflictions are many, He hears their cry and delivers them from them all. Quite the opposite, He cuts off the remembrance of the wicked from the Earth. Third, it is the temptation that will happen to the Earth after believers are raptured out (Rev. 3:10). This temptation will be unlike any other in magnitude. This is the future judgment that is coming to all these ungodly men who deny the Lord God that bought them, but we as believers will not experience this.