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]
The promise in the verse refers to the second coming of Christ, an event the early church anticipated, but did not happen during their lifetime. When we read the bible, we see God foretelling and fulfilling His promise regarding the delivering of Israel out of Egypt and, later, Babylon, and the birth of Christ. Thus we can be sure He will fulfill His promise regarding the second coming. Peter explained to the readers that they should not mistake the delay as slackness – it is due to His patience and grace that He is delaying the promise, giving more time for the human race to repent.