2 Peter 3:8

But, beloved, be not ignorant of this one thing Literally, the construction being the same as in 2 Peter 3:5, let not this one thing be hidden from you.

that one day is with the Lord as a thousand years The latter clause has its origin in the words of the Psalmist, "A thousand years in thy sight are but as yesterday" (Psalms 90:4); but while the Psalmist dwells only on the littleness of our greatest time-measures, the Apostle completes the thought by joining with it the possible greatness of that which to our sight is almost infinitely little. "A day" (probably with special reference to the day of judgment) may be pregnant with results for the spiritual history of mankind or of an individual soul as great as those of a millennium. The delay of a millennium may be but as a day in the evolution of the great purposes of God. The words have the additional interest of having impressed themselves as a "faithful saying" or axiom of religious thought on the minds of the apostolic age, and are quoted as such in the Epistle that bears the name of Barnabas (chap. 15). This forms the second answer of the Apostle to the sneering question of the mockers.