3:2 Our minds become subject to a purifying effect as we "take heed" to prophecy of the old Testament. Peter stresses such importance in chapter one, that we do well if we "take heed as unto a light that shineth in a dark place" [2Peter 1:19]. If ever the world is in a "dark place", it is now!

What comfort is afforded a believer in the prophetic truth and proof (in the Old Testament) of the first advent of Christ into the world; and what great help for a "pure mind" is all the prophecy in the New and also the Old Testament of the glorious return of the Lord as "a thief in the night". Such truth has a purifying affect upon any thinking and temptation of the flesh and the world!
The apostle John says the same thing.. .... "Beloved, now are we the sons of God, and it doth not yet appear what we shall be: but we know that, when he shall appear, we shall be like him; for we shall see him as he is. And every man that hath this hope in him purifieth himself, even as he is pure. [1 John 3:2-3]

The unexpected appearence of the Lord promotes purity of mind and attitude towards the World and our living in it, and not forgetting the all important "pure mind" towards the saints as well, so that we are not "ashamed before him at his coming" [1 John 2:29].

3:3 Important to Know.

The sad prediction is, that what was a world of fair consideration about the things of God is now a world of scoffing and ridicule of the Bible and the Gospel of Christ.
3:4. What fun is had among the "scoffers" with such words as ... "O you Christians have been prattling on and on, since the dark ages about Jesus coming back; So where is he"?? But unfortunately the joke is upon them, to the peril of their life!

3:5. The poor athiests are willingly ignorant of two important things ...
1). The sheer mind boggling precariousness of the earth "standing out of the water and in the water", of which it can only be concluded, that for such a dangerous entity to be held without disaster for thousands of years, must have been established and held now in continuence, by "the Word of God"!
2). The Lord then doubled up the seemingly impossible, by destroying the Earth (Noah's flood - Vs 6 ... "Whereby the world that then was, being overflowed with water, perished.") through a collapse of all the divinely decreed limits of the waters under the Earth and above the Earth.

In the following, God challenges Job that by His decree alone, the waters of the Earth are controlled.
..... "And said, Hitherto shalt thou come, but no further: and here shall thy proud waves be stayed?" [Job 38:11].