Answer for those who contend that the people referred to in 2 Pet 2:20-22 were never saved
Of course the immediate context is speaking about false teachers. However, verse 20 is not referring to the false teachers, but to those who "escaped from them that live in error" (v18b), whom the teachers were trying to allure (v18a). Peter claims at the beginning of the epistle, that Christians are saved "through the KNOWLEDGE of Him [Christ]" (2 Pet 1:3) and they have "ESCAPED the corruption that is in the world through lust" (1:4). Then in chapter two, which we are addressing, Peter clearly speaks of the same type of person falling away (due to the instigation of the false teachers) and having a worse punishment than an unbeliever (2 Pet 2:20-22). This person had previously "ESCAPED the pollutions of the world through the KNOWLEDGE of the Lord and Savior Jesus Christ" (2:20). Both of these characteristics were the same ones listed at the beginning of the epistle concerning Christians (1:3-4). How much clearer could it get?
KNOWLEDGE of Christ (Greek epignosis- literally "exact knowledge") is always associated with saving knowledge (Rom 10:2, Eph 1:17, 4:13, Col 1:10, 1 Tim 2:4, 2 Tim 3:7, Heb 10:26, 2 Pet 1:2-3, 1:8, 2:20). Paul tells Timothy that those who are saved have come to "the knowledge of the truth" (1 Tim 2:4), and those who are unsaved are "never able to come to the knowledge of the truth" (2 Tim 3:7). Jesus says "This is life eternal, that they might KNOW thee..." (Jn 17:3). So as adamant as eternal security advocates about their prooftexts, they seem to have very wanting explanations for this very clear cut passage.
Other Apostasy Passages
I would ask every Calvinist to be honest about clear apostasy texts, such as Heb 10:26, 6:4-6, 3:12, John 15:2, Gal 1:6, etc. I would also ask them to go and re-analyze their supposed "eternal security" proof texts and notice that many of them are stated in the following format: "I am persuaded (confident) that........" (Rom 8:38, Heb 6:9, Php 1:6, 2 Th 3:4, 2 Tim 1:12). A statement of confidence on Paul's part should never be considered an unconditional promise. This is a very weak hermeneutic. These "confidence" statements of Paul should not be given a lot of doctrinal weight. This would be like creating a doctrine "Planes never crash" because you heard someone tell their loved one before they got on the plane, "I'm confident your plane will land".
A more obvious hermeneutic would place more weight on the countless warnings given to believers throughout every epistle. The epistles were written to "saints" -- not a mixed audience of saints and sinners (as Calvinists advocate). In Colossians 1, Pauls writes to "the saints and faithful brethren which are in Christ". Simple hermeneutics tells me that anytime Paul uses the word "You" or "Ye" in Colossians, he is referring to the "saints and faithful brethren which are in Christ". Notice what he promises the "Ye":
"21 And you, that were sometime alienated and enemies in your mind by wicked works, yet now hath he reconciled 22 In the body of his flesh through death, to present you holy and unblameable and unreproveable in his sight: 23 If ye continue in the faith grounded and settled, and be not moved away from the hope of the gospel, which ye have heard, and which was preached to every creature which is under heaven;"
Now, either Paul was throwing out a completely artificial warning "if ye continue in the faith", or he really meant "if ye continue in the faith". I could present countless examples, but this one will suffice to show that Paul taught the possibility of apostasy.
Most of Paul's other "unconditional" promises about security are qualified with the phrase "in Christ" (e.g. Rom 8:1, 39), which implies identification with Christ through faith. We are only "In Christ" by actively abiding in Him by faith (See John 15). It is only by faith that we are "in Him". Other promises, which may even be written seemingly unconditionally (e.g. Rom 8:30), should always be tempered by conditions stated elsewhere in scripture. For even Joshua was given a promise of God's presence (seemingly unconditionally), yet we later see that the promise was clearly conditional (Compare Joshua 1:5,9 with Joshua 7:12).