2 Peter 3:15-16: "And count the patience of our Lord as salvation, just as our beloved brother Paul also wrote to you according to the wisdom given him, as he does in all his letters when he speaks in them of these matters."
We need to understand that the reason Christ has not returned yet is to give time for the full number of God's people to be saved. 2 Peter 3:15a is a recap of 2 Peter 3:9. Peter is saying that the Lord is not unfaithful to His promise. The very reason why Christ is not coming back yet is to give time for the people of God to repent and be saved. Through the proclamation of the gospel, God is patiently calling His people from all over the world to repent from their sins and return to Him in faith.
Verse 15 is God's word on how to interpret the time between the first and the second coming of Christ. It is the time of salvation. The Savior has come and has opened the way for God's people, His sheep, to return to His fold. While our Lord Jesus Christ has not appeared for the second time, the way to God is still open. But when He comes back the way will be closed and the time of salvation will be over.
Notice that, according to Peter, this is also what Paul taught and wrote in his letters. Peter says, “And count the patience of our Lord as salvation.” Paul says in Romans 2:4, “Or do you think lightly of the riches of His kindness and tolerance and patience, not knowing that the kindness of God leads you to repentance?”
So both the Apostles Peter and Paul teach that God's postponing or withholding final judgment on the wicked is an act of forbearance that should be interpreted as 'giving more time', so to speak, for our repentance and salvation.
Moreover, in 2 Cor. 6:2 Paul also says, “And working together with Him, we also urge you not to receive the grace of God in vain — for He says, "AT THE ACCEPTABLE TIME I LISTENED TO YOU, AND ON THE DAY OF SALVATION I HELPED YOU." Behold, now is "THE ACCEPTABLE TIME," behold, now is "THE DAY OF SALVATION"” (emphasis mine).
So by mentioning Paul and his letters, Peter shows that there is agreement among the apostles. The false teachers may reject the perfect law of God or deny the second coming of Christ, but the apostles of Jesus are united on these matters: Christ is coming again and you should live a life separated from sin and consecrated to God. Meanwhile His delaying is for our salvation.
The knowledge that all the apostles of our Lord Jesus Christ are united in their doctrine is really enriching. It helps us to see how God divinely superintended the writings of the Holy Scripture ( 2 Peter 1:21) so that when it is correctly interpreted and applied to us now, it confirms the divine authority of God. And it accomplishes the purpose for which it is intended for the hearers – that is, their salvation and sanctification, and for God – His praise and glory. Again verse 18 tells us, “...but grow in the grace and knowledge of our Lord and Savior Jesus Christ. To Him be glory, both now and to the day of eternity.”
God's patience is God's work of grace in order to give us time to repent, believe and be saved. In God's sovereign plan, it is also the time for us to grow in our knowledge of Him and to be continually transformed by that knowledge from the inside out so we can progress in holiness.
That's why we take this time as opportunity to spread the good news of Jesus Christ through out the world, to call people to repentance and faith and to disciple the nations of the world, teaching them everything Christ has commanded us to do. You grow in the grace of God when you participate in taking this opportunity to go out to your neighborhood and proclaim the mercy and grace of our God and, at the same time, warn them of the coming judgment if they persist in unbelief.
You also grow in the grace of God when you take your sin seriously. Sin is an offense to the holy character of God. When we sin we commit something that the law of God forbids and God will not let us go away with that, unless we repent and depend solely on the grace of God.
Will you forsake your sin and serve the Lord while the way is still open? Do not provoke the Lord's patience by persisting in disobedience. But in humility, confess your sin and trust in the finish work of Christ, the blameless and spotless lamb of God. He will truly save you until the end.