Question: Could you explain 1st Peter 2:12 for me?
Response: As to 1st Peter 2:12, you weren't specific in your question about the passage - I assume that your question has to do with why unbelievers would consider Christians "wrong-doers"?
Keep your manner of life among the gentiles (i.e., unbelievers) [morally] good, so that when they slander you as evil-doers they may [yet] give glory to God when they look upon your good works (i.e., life and production).
1st Peter 2:12
From the historical point of view, Christians 1) didn't do what everyone else did, and 2) did things no one else did. Both of these general behavior patterns are often enough to arouse suspicion, envy, hate, etc. Christians abstained from the excesses of the pagan world (fornication, foul language, etc.), on the other hand they were honest, gracious, not hyper-competitive, - in short, were virtuous in their approach to life because of their love of and response to the truth of God's words. Peter remarks elsewhere that this restraint from the ways of the world was a cause of amazement on the part of their pagan neighbors - and of slander:
For you have spent enough time in the past doing what the pagans choose to do - living in debauchery, lust, drunkenness, orgies, carousing, and detestable idolatry. They think it strange that you do not plunge with them into the same flood of dissipation, and they heap abuse on you.
1st Peter 4:3-4 NIV
Add to this that, of course, Christians were not willing to participate in the day-to-day pagan rituals of their communities. Generally speaking, most "religions" of the world have been eclectic and willing to accept many additions. Christianity, however, is exclusive as the only true way to God, and will not admit compromise (Ex.20:3; Jn.14:6). This will have been a particular thorn in the side of the unbelieving population of the various cities of Asia Minor to whose Christian population Peter is writing. On the second point, Christian worship was not temple and sacrifice centered or composed of the same sort of mumbo-jumbo as the usual pagan rites. It was conducted in private homes, and the communion ceremony in particular could easily be misconstrued (and apparently was) by those with mal-intent as some sort of bizarre ritual - the sort of thing that conspiratorial secret societies indulge in. Although the Roman Empire produced a time of peace and prosperity unheard of elsewhere in the ancient world, it was not technically permitted to have a religion or to worship in a non-traditional way that had not been approved; and although Judaism was a so-called religio licta, Christianity was not. As soon as it became clear to Roman officialdom that Judaism and Christianity were not the same thing, Christianity came in for the persecution that characterized the Smyrnan era of the Church (see Coming Tribulation part 2A: The Seven Churches, "Smyrna").
All this has important application today. It really does seem to be getting to the point in our society (not to mention the rest of the world where things are worse) that Christians who really do take seriously the mandate from our Lord to sanctify ourselves from the evil practices of the world are viewed with jealousy and suspicion, and not a little skepticism. This is emanating as much from so-called "fellow Christians" in various denominations as much as it is from unbelievers. That is a real foretaste of the Great Apostasy and Great Persecution to come. And not only that, but it is becoming increasingly politically incorrect for true Christians to stand true to their principles. When, for example, we point out to those who ask that one can only be saved through faith in and faithfully following Jesus Christ, we are seen as being intolerant of other faiths (or, increasingly, of other interpretations of "Christianity") - and this is true of course of any principle of truth we may express which is clearly not the popular point of view. Not only that, but there is of course abroad today a spirit of aggressive anti-Christianity which seeks to make it impossible to express one's faith in anything but private without paying a price. But everyone who is truly seeking to walk with Jesus day by day will continue to put His truth, the Word of God, in first place in their lives, studying the Bible, seeking out solid teaching, believing what the scriptures say, and putting the principles of truth it contains into practice in every aspect of their lives, and ministering to others to the end that they may be helped to do the same.
I find it very disheartening that this battle is currently being fought out in superficial and highly politicized areas (one only needs to listen to the news) and that the real point is being missed: if we want to follow Jesus in the way He prescribed and witness to Christ in the way we must, then we run the great risk of being socially and sometimes palpably penalized to an extent that was not true even a few short years ago. Clearly, this trend of demonizing true believers who are truly dedicated to the principles of truth in the Word of God is accelerating dramatically, and many true believers seem unequal to the task of endurance and faithful witness (to judge by the political responses to which many have succumbed when a spiritual response is really what is needed and indicated). This is, in my view, symptomatic of a failure on the part of the bulk of the believers in our Laodicean era (see Coming Tribulation part 2A: The Seven Churches, "Laodicea") to put the Bible and its diligent study in any meaningful place in their list of priorities. It is hard to do what is right if one does not really know what is right. And I think it is beyond question as true today as it was in Peter's day that those of us who put the diligent and systematic study of the Bible and its truth in the place of honor in our spiritual walk (and truly do so through more than just lip-service) are indeed regarded as "strange".
This brings me to my last point of reference between the ancient and the modern world. Peter's advice in the verse you ask about is more pro-active than defensive. There may be little we can do to assure suspicious or implacable opponents that our faith is truly good and pure, but we can certainly demonstrate by our lives and our deeds that the outcome of the things we profess is both innocent and intrinsically good in every way. And being effective in powerful witness of the life such as Peter encourages us to lead is directly tied into "doing what the rest of the world does not" (and, increasingly, what many so-called Christians do not). For all genuinely "good works", come as a result of true spiritual growth; that is, from hearing, believing, and applying the Word of God. For it is from the truth, its acceptance and profession, that all genuine deeds of love truly flow. Doing a "good thing" from the wrong motivation, seeking to "help God" rather than respond to Him in truth, is a pointless exercise, after all.
Would that it really was our genuine Christian purity and selfless good deeds producing our alienation from the world, because learning and living the truth is by far the best way we have of shining light on its darkness and reproving its deeds. Sadly, when collectively all the visible church can muster lacks spiritual substance and is merely for show, and all she can do to respond to the world's suspicions is to form political movements, then there is not much to hope for from the traditional establishment (a sad conclusion that many of us have long since drawn). But as individual, dedicated followers of Jesus Christ, we know that the challenge of personal, daily spiritual growth, and dedicated ministry to the Body of Christ in accordance with the gifts we have been given must be met day by day as we continue to pick up our cross and follow Him.
"[Look] to the Law and to the Testimony" - if they do not speak according to this word, it is because there is not [light of] dawn in them.
In Him who is able to keep us from falling and bring us safe to our heavenly abode, the One who has known us from all eternity and stands at our side, ready to help at all times, our Lord and Savior Jesus Christ.