...Isa. 55:1; Jer. 13:17; Ezek. 18:32; Matt. 11:28; 23:37; Luke 13:3; John 3:16; 8:21, 24; 1 Tim. 2:3–4; Rev. 22:17). all should reach repentance
. “All” (cf. “you,” “any”) must refer to all who are God’s people who will come to Christ to make up the full number of the people of God. The reason for the delay in Christ’s coming and the attendant judgments is not because he is slow to keep his promise, or because he wants to judge more of the wicked, or because he is impotent in the face of wickedness. He delays his coming because he is patient and desires the time for his people to repent.
The ESV Reformation Study Bible explains: 
3:9 as some count slowness. See v. 4.
patient . . . all should reach repentance. Peter’s Christian readers must realize that the apparent delay of divine judgment is a sign of God’s forbearance and mercy toward them, particularly toward the believers in their midst who have been confused and misled by the false teachers. The repentance in view, for the sake of which God delays judgment, is that of God’s people rather than the world at large. God is not willing that any of His elect should perish (John 6:39).
The HCSB Study Bible explains: 
3:9 The Lord has not yet returned, says Peter, because He is patient with you, not wanting any to perish. "You" is variously interpreted as a reference to the letter's Christian recipients (identified in 1:1) or else more broadly as all people. In chapter 1 "you" and "your" both refer back to the recipients identified in 1:1 (see 1:2,4,5,8,10,11,12,13,15,16,19,20). Peter's later use of "dear friends," (3:1,8,14,17) seems also to point back to those identified in 1:1.
What Matthew Henry said about 2 Peter 3:9: 
That what men count slackness is truly long-suffering, and that to us-ward; it is giving more time to his own people, whom he has chosen before the foundation of the world, many of whom are not as yet converted; and those who are in a state of grace and favour with God are to advance in knowledge and holiness, and in the exercise of faith and patience, to abound in good works, doing and suffering what they are called to, that they may bring glory to God, and improve in a meetness for heaven; for God is not willing that any of these should perish, but that all of them should come to repentance. Here observe, 1. Repentance is absolutely necessary in order to salvation. Except we repent, we shall perish, Luke 13:3; Luke 13:5. 2. God has no delight in the death of sinners: as the punishment of sinners is a torment to his creatures, a merciful God does not take pleasure in it; and though the principal design of God in his long-suffering is the blessedness of those whom he has chosen to salvation, through sanctification of the spirit, and belief of the truth, yet his goodness and forbearance do in their own nature invite and call to repentance all those to whom they are exercised; and, if men continue impenitent when God gives them space to repent, he will deal more severely with them, though the great reason why he did not hasten his coming was because he had not accomplished the number of his elect. "Abuse not therefore the patience and long-suffering of God, by abandoning yourselves to a course of ungodliness; presume not to go on boldly in the way of sinners, nor to sit down securely in an unconverted impenitent state, as he who said (Matt. xxiv. 48), My Lord delayeth his coming, lest he come and surprise you;"
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