2 Peter 2:1
But false prophets also arose among the people, just as there will also be false teachers among you, who will secretly introduce destructive heresies, even denying the Master who bought them, bringing swift destruction upon themselves.
While this is a difficult text, it is actually quite ambiguous. The case against limited atonement from this verse is not as great as it seems.
First, it is unclear exactly what Peter means when he says the false teachers were "bought." It is true that that 1 Corinthians 6:20 and other verses use "bought" as a reference to what Christ did at His death. But that does not mean that the word is used in this way everywhere it appears in Scripture.
As John Owen points out in The Death of Death in the Death of Christ, the word used to say the false teachers were "bought" can be used to denote any kind of deliverance, and so does not necessarily indicate that they had been purchased by the blood of Christ. Based on the context, it may be best to understand the statement that the false teachers had been "bought" not as a reference to the death of Christ, but a reference to some other act of deliverance--such as deliverance by God's goodness from the idolatry of the world. Notice how later on Peter refers to the false teachers as having had a form of "deliverance" in that they "escape[ed] the pollution of the world" by the knowledge of the gospel (v. 20). This verse is not referring to salvation, but outward reformation with no ultimate inward reality. These people did not have their natures changed and so returned to the mud like a pig. We all know of many unsaved people who for a time reform their lives, but soon go back to their old way. In 2:20 Peter is saying that the false teachers are like that; and so in 2:1 it is possible that the "deliverance" or "purchase" of these teachers refers to their outward escape from the pollution of the world and thus does not imply anything about whether Christ had bought them by His death.
There is also another possibility. Wayne Grudem makes a good case that Peter is referring to the Exodus in 2:1. For Peter compares the false prophets that would arise in the church to the false prophets that arose in Israel: "False prophets also arose among the people, just as there will be false teachers among you." In the Old Testament the whole nation of Israel, and thus even the false teachers in it, was considered to have been "bought" by God in the Exodus from Egypt. Through this deliverance, God "bought" the nation of Israel and thus Israel rightfully belonged to God as His peculiar people. We see this in Deuteronomy 32:6, which is the passage that Peter is probably alluding to: "Do you thus repay the Lord, O foolish and unwise people? Is not He your Father who has bought you? He has made you and established you." God "bought" Israel not by the death of Christ but, as this text says, by forming the nation. This is evident from Exodus 15:16 as well, which speaks of the Exodus as the act of God whereby He "bought" Israel: "Terror and dread fall upon them; by the greatness of Thine arm they are motionless as stone; until they people pass over, O Lord, until the people pass over whom thou hast purchased."
So the nation of Israel was considered "bought" by God because of the Exodus. Since 2 Peter 2:1 is comparing the false teachers who arise in the church with the false prophets who arose in Israel, could it not be that Peter is saying that these false prophets will be from the nation of Israel--that is, those who were "bought" in the Exodus? Or, perhaps could he not be saying that these false teachers will be church attenders in a position analogous to those in Israel who had been "bought" at the Exodus?
Regardless, we see that there are many different things Peter could mean when he says the false teachers were "bought" by the Lord. Because of this ambiguity, it would not be wise to take this as a passage denying limited atonement. In fact, in light of the clear teaching elsewhere in Scripture that limited atonement is true, it would be best to interpret this ambiguous passage in light of those.
Second, it is also ambiguous whether Peter is referring to God the Father or Christ as the Lord who bought them when he says that they will "even deny the sovereign Lord who bought them." In fact, it is likely that the "sovereign Lord" who Peter says had bought these false teachers is a reference to God the Father, not Christ. This is because in the following verses God the Father is spoken of and the Greek word for Lord used here is never used of Christ, but only of the Father (see John Owen's The Death of Death in the Death of Christ). This understanding is also most in line with the allusion to Deuteronomy 32:6, where God the Father is in view and is said to have "bought" Israel.
If Peter is saying that God the Father bought these false teachers, it cannot be a reference to the atonement. Why? Because the atonement was made by Jesus, not the Father. Thus, here is another reason that it is likely that the purchase spoken of here is not a reference to the death of Christ.
Third, it is ambiguous whether Peter is speaking of the reality of a purchase, or according to the appearance of a purchase--that is, their outward appearance and profession. In other words, the verse may mean, "denying the Master who [they say] bought them [but really didn't]," or it may be intended to confirm that these false teachers would come from within the visible church. To speak of them as "bought," then, wouldn't mean that Christ had died to save them, but that they occupied a position that is supposed to be occupied only by those who have been bought.
So we have seen that there are three large ambiguities in 2 Peter 2:1. First, it is unclear whether the purchase of these false teachers is a reference to the death of Christ or not. Second, it is unclear whether the one who "bought" them is even Christ or simply the Father. Third, it is unclear whether Peter is speaking according to reality or appearance.
Because of these huge ambiguities in 2 Peter 2:1, it is not a solid text against limited atonement. There are many things it could legitimately mean, and so it would not be wise to stand on it as an argument against limited atonement.