"The Lord is not slow to fulfill his promise as some count slowness, but is patient toward you, not wishing that any should perish, but that all should reach repentance."
2 Peter 3:9
Peter continues to answer the mocking of the false teachers working among the Christians in the early church. They ask, "Where is this coming Jesus promised?" They teach that it's been too long; Christ is not coming. Don't resist immorality; there will be no judgment. In verse 8, Peter urged his readers to remember that the Lord is not bound by human time. For God, a thousand years is like a day and vice versa. Peter's point is that God does not suffer the limitations of time, or confusion about it, the way human beings do.
Here in this verse, Peter insists that we cannot apply human demands about time to the promises of God. He is not slow in keeping His promise. God is the one who made the schedule: He cannot be "late." Instead, God keeps every promise at the perfect time for His glory and for the good of those He loves.
In this case, Christians should view the delay in Christ's return as evidence of God's patience, not of His tardiness. In His love-driven patience, God is willing to give more time for more people to come to repentance. This is God's plan to allow more people opportunity to place their trust in Christ in order to enter into eternal relationship with Him.
God doesn't want anyone to perish or die. Peter likely refers to eternal death following God's judgment on the day of the Lord. The overall message of Scripture is that God does not desire anyone's damnation. That is, He would prefer that all would be saved. However, in His sovereignty and power, God decided not to demand—force—all people to actually be saved. If God is truly sovereign, He can sovereignly allow us to choose things He does not prefer, for His own reasons. Here, Peter shows us God's heart for the people He has created: He wants them all to be saved, but He will not force them all to be saved.
As Peter tells us, that's one reason God allows more time—the very time mocked by the false teachers—prior to the return of Christ. He is mercifully creating more space for more people to repent and turn to Him.