"The Lord is not slow about His promise, as some count slowness, but is patient toward you, not wishing for any to perish but for all to come to repentance" (2 Peter 3:9).
In the context surrounding this verse, Peter is talking about the coming day of the Lord. This judgment would not come as quickly as some thought it would. This caused some to mock the idea and believe it will never happen (2 Peter 3:3-4).
The length of time until the day of the Lord is not because God is slow about His promise. After all, "with the Lord one day is like a thousand years, and a thousand years like one day" (2 Peter 3:8). God is not bound or regulated by time like we are. The wait until that day is not due to God's slowness, but God's patience.
Why is God being patient? Because He is "not wishing for any to perish but for all to come to repentance." God is not just waiting to do something. He is waiting for us to do something. In delaying judgment, He is giving us time to repent and get our lives right before Him.
However, while we have time now, we will not always have time to repent. "But the day of the Lord will come like a thief, in which the heavens will pass away with a roar and the elements will be destroyed with intense heat, and the earth and its works will be burned up" (2 Pet. 3:10).
We must not think lightly of God's patience or take it for granted. He has been gracious to give us the time we have to serve Him. Let us be ready to meet Him so we will not be caught unprepared.