2 Peter 3:8

The apostle comes in these words to instruct and establish Christians in the truth of the coming of the Lord, where we may clearly discern the tenderness and affection wherewith he speaks to them, calling them beloved; he had a compassionate concern and a love of good-will for the ungodly wretches who refused to believe divine revelation, but he has a peculiar respect for the true believers, and the remaining ignorance and weakness that the apprehends to be in them make him jealous, and put him on giving them a caution. Here we may observe, I. The truth which the apostle asserts - that with the Lord one day is as a thousand years, and a thousand years are as one day. Though, in the account of men, there is a great deal of difference between a day and a year, and a vast deal more between one day and a thousand years, yet in the account of God, who inhabits eternity, in which there is no succession, there is no difference; for all things past, present, and future, are ever before him, and the delay of a thousand years cannot be so much to him as the deferring of any thing for a day or an hour is to us. II.

— Matthew Henry's Concise Commentary (public domain)

be not ignorant--as those scoffers are (Pe2 3:5). Besides the refutation of them (Pe2 3:5-7) drawn from the history of the deluge, here he adds another (addressed more to believers than to the mockers): God's delay in fulfilling His promise is not, like men's delays, owing to inability or fickleness in keeping His word, but through "long-suffering." this one thing--as the consideration of chief importance (Luk 10:42). one day . . . thousand years-- (Psa 90:4): Moses there says, Thy eternity, knowing no distinction between a thousand years and a day, is the refuge of us creatures of a day. Peter views God's eternity in relation to the last day: that day seems to us, short-lived beings, long in coming, but with the Lord the interval is irrespective of the idea of long or short. His eternity exceeds all measures of time: to His divine knowledge all future things are present: His power requires not long delays for the performance of His work: His long-suffering excludes all impatient expectation and eager haste, such as we men feel. He is equally blessed in one day and in a thousand years. He can do the work of a thousand years in one day: so in Pe2 3:9 it is said, "He is not slack," that is, "slow": He has always the power to fulfil His "promise.

— Jamieson-Fausset-Brown Commentary (public domain)