Peter urged his readers to not only be mindful of these scoffers but to be mindful of the Scriptures. The Scriptures are critical in guarding believers against false teaching. So to his exhortation to be mindful, Peter now exhorts his reads to be not ignorant of two great truths -- the doctrines of God and the End Times. These two great doctrines were under attack by the false teachers.
The question regarding the promise of the Second Coming is an attack on the End Times doctrine. This doctrine is known as Eschatology. The question is also an attack on the doctrine of God, known as Theology Proper. The prophecies of Christ’s Second Coming are promises made by God, Who guarantees their fulfillment. By questioning the completion of these promises, the false teachers made God out to be a liar. As well, they claimed that God was uninvolved in His creation. Such a view is known as deism. So in 2 Peter 3:8-13, Peter exhorts believers to be not ignorant of two great truths!