"The Lord is not slow about His promise, as some count slowness, but is patient toward you, not wishing for any to perish but for all to come to repentance." -2 Peter 3:9
A quick citing of the second part of this verse is usually enough for my non-reformed brethren, "not wishing for any to perish but for all to come to repentance." It's usually enough, it is thought, to put that annoying Calvinist who likes to argue in his place. This will not do if we are ever going to let the text speak for itself. The non-Reformed usually assume that "any" and "all" refers to all of the human race. When the context is taken into account this is shown not to be true. 2 Peter 3:9 is a text that strongly supports the Reformed position and not the non-Reformed position. Adding bold to the following text will help to make this point.
"Know this first of all, that in the last days mockers will come with their mocking, following after their own lusts, and saying, "Where is the promise of His coming? For ever since the fathers fell asleep, all continues just as it was from the beginning of creation." For when they maintain this, it escapes their notice that by the word of God the heavens existed long ago and the earth was formed out of water and by water, through which the world at that time was destroyed, being flooded with water. But by His word the present heavens and earth are being reserved for fire, kept for the day of judgment and destruction of ungodly men. But do not let this one fact escape your notice, beloved, that with the Lord one day is like a thousand years, and a thousand years like one day. The Lord is not slow about His promise, as some count slowness, but is patient toward you, not wishing for any to perish but for all to come to repentance. But the day of the Lord will come like a thief, in which the heavens will pass away with a roar and the elements will be destroyed with intense heat, and the earth and its works will be burned up. Since all these things are to be destroyed in this way, what sort of people ought you to be in holy conduct and godliness, looking for and hastening the coming of the day of God, because of which the heavens will be destroyed by burning, and the elements will melt with intense heat! But according to His promise we are looking for new heavens and a new earth, in which righteousness dwells." -2 Peter 3:3-13
It should be noted that the context of this passage is speaking of the second coming of our Lord, salvation is mentioned in passing. When reading this text it is obvious that there are two different groups of people being addressed, "Mockers"(unbelievers) and the ones "looking for new heavens and a new earth"(believers). The "Mockers" are making fun of believers because Jesus hasn't come back yet, much like non-believers do today. But Peter is quick to point out that God's time isn't like our time. The "your" in "do not let this one fact escape your notice" and "beloved" in V.8 are addressed to Christians. This gives us the meaning of: "you," "any," and "all" in V.9. In V.9 When Peter says that God "is patient toward you, not wishing for any to perish but for all to come to repentance," he is referring to Christian's who have the promise of Jesus' second coming and of the new heavens and new earth. In 2 Peter 1:1 Peter tells us the audience to whom he is writing, "...to those who have received a faith of the same kind as ours..." God's elect are the audience of Peters previous letter as well, 1 Peter 1:1 "...to those who reside as aliens...God's chosen." The contrast between Christian and non-Christians informs us on how Peter uses "any" and "all" in V.9 as well as his use of "patient toward you." The use of "you" in V.9 is obviously a reference to Christians. This is especially the case since only Christian's have the promise of His coming, and the fact that God's patience is only towards Christians. Thus the "any" and "all" in V.9 are God's elect who God will certainly bring to repentance without fail.
The non-Reformed interpretation of 2 Peter 3:9 has God trying to save as many as He can, but is frustrated since man will not cooperate with His saving grace, as if God's grace needs man's cooperation in order to be effective. God's effectual grace is a powerful force and cannot fail. Rather then contradicting the Reformed position 2 Peter 3:9 affirms that God is not willing that any of His elect should perish. Peter tells his audience that Jesus' second coming has not been delayed but that God is patiently gathering His chosen people. God isn't trying to bring people to repentance and then being frustrated by the will of Man. God does require repentance for all of His creatures but He only grants it to His chosen people by His sovereign hand.