I urge you, brothers, to watch out for those who cause divisions and put obstacles in your way that are contrary to the teaching you have learned. Keep away from them. For such people are not serving our Lord Christ, but their own appetites. By smooth talk and flattery they deceive the minds of naive people. Everyone has heard about your obedience, so I am full of joy over you; but I want you to be wise about what is good, and innocent about what is evil. (NIV)
First I note that this is very important to him. He uses very strong language to urge them to comply with what he has to say in the next couple verses. It would not be a stretch to translate this as I'm begging you! Next is to watch out for and the NIV rendering if this might be a bit weak as the original language here can also be translated scrutinize, which implies a very close look indeed, not just a casual alertness. Following this, the word translated as divisions implies a wrongful separation, not one of propriety or necessity and the word which comes through as obstacles is the word from which we get scandal. (By the way, all this comes from a very simple, almost superficial look at the original language with only a few easily available study tools.)
The teachings you have learned can be summed up in what Paul had already written in that letter. He'd given them a short course in systematic theology in the pages they'd just read: Wrath, righteousness, sin, punishment, grace, redemption.... It had all been summed up in the previous 15 chapters including how all of that should play out in their everyday lives, umm, make that our everyday lives also.
In spite of their appearance, those people about whom Paul warned were serving only themselves, not the cause of Christ. They were deceiving unknowing people and Paul wanted his readers to not be taken in by them. All that is still ongoing today. In our churches we see these things that are killing the cause of Christ:
A primary problem is a corrupted clergy. Not long ago, Tufts University university did a survey of non-believing pastors who were still in the ministry; the very thought of that should be chilling to believers. That a survey could find enough such folks to participate suggests to me that those they studied represent the tip of the iceberg. Too many seminaries have dropped their standards, not holding to the absolutes of Scripture. The next result is a diluted doctrine. As a result of what's being taught the clergy, all has become relative. The purpose of Christ--if not the fact of Christ--has been lost. He came to seek and save the lost. Though He did many other things along the way, those were not the purpose for which he came and churches have turned that around. Churches may do good works; but they've forgotten the lost. Sadly, it appears that they are lost themselves.
This gives us losers for leaders. I'm aware of a church leader who has said that he "has no time for this God stuff," and that it's all about "butts in the seats and dollars in the plate." Such are those elected to leadership. Our churches now face a failing future. Look at them, many older folks, a few middle aged and a handful of young children with their parents--or generally with their mothers, as Dad can't be bothered.
Paul warned in the text that they deceive the minds of naive people. As the church leaders ceased believing and teaching the Word of God and it's absolute authority, the level of Biblical knowledge has declined to the point where it is nearly unknown. This would be sad enough, but we have been warned! Paul warned us here; Peter wrote of similar things as did Jude. Why don't we get it?