There are lots of different kinds of false teachings around in the world today. There are teachings like Mormonism, Jehovah's Witnesses and other cults which add to and subtract from the Word of God in order to create a new religious empire. There are really off-base occult philosophical systems like Theosophy, Rosicrucianism and the smorgasbord of various New Age systems including Hinduism and Buddism which some people select ideas from in order to construct their own personal belief system. All these things take people away from Jesus Christ. A real Christian is not likely to be deceived by such things.
Peter spent time in his letter, as did Jude the brother of the Lord in his, to warn REAL CHRISTIANS about the very real danger of false teachers. They were not so likely to be deceived by the above kinds of heresies, though Gnosticism was (and is) a dangerous heresy with a strong appeal to human pride which deceived many.
The kind of false teachers much more likely to deceive real Christians (and yes, real Christians can potentially be deceived) is the teacher who talks about Jesus, and teaches about the blood of Jesus, and the grace of God, but lives an immoral greedy life and virtually encourages his hearers to do the same.
Satan can transform himself into an angel of light, and his servants can transform them into "ministers of righteousness".
2Co 11:13 For such are false apostles, deceitful workers, transforming themselves into apostles of Christ.
2Co 11:14 And no wonder! For Satan himself transforms himself into an angel of light.
2Co 11:15 Therefore it is no great thing if his ministers also transform themselves into ministers of righteousness, whose end will be according to their works.
There has been so much anti-biblical teaching that implies that you can live a wicked life and still go to heaven, that it needs to be pointed out that even people who attain to church pulpits will be judged by God. They may appear to be apostles of Christ, ministers of righteousness, or whatever, but if they are fall into any of the categories of "immoral" or "greedy" they WILL BE DAMNED (unless they repent).
Paul says clearly "their end will be according to their works". Not according to the blood of Jesus. The blood of Jesus cleanses those who REPENT and trust God, not those who persist in wilful sin. See 1 John 1:7.
Peter says the same thing.
2Pe 2:4 "For if God did not spare the angels who sinned, but cast them down to hell and delivered them into chains of darkness, to be reserved for judgment; …
2Pe 2:9 then the Lord knows how to … reserve the unjust under punishment for the day of judgment,
2Pe 2:10 and especially those who walk according to the flesh in the lust of uncleanness and despise authority. They are presumptuous, self-willed. "
Remember that Peter is talking about False Teachers who will be among you (vs 1).
Some have this idea that God is so warm and fuzzy that he would never hurt or judge anyone. Sorry, but that isn't the God of the Bible. Read it carefully. We serve a God who violently HATES sin, and though he patiently endures it for a season, and gives people time to REPENT (Revelation 2:21), ultimately the judgement will come, and it will involve eternal hellfire.
Is this legalism? No, this is the BIBLE. Legalism, lets be clear, is the idea that striving to attain to a system of rules will earn you merit with God through which you will be saved. God's system is rather that we renounce ourselves – our own righteousness, our own power, and be filled with His Spirit, guided by Him directly, so that we have a changed life. Such people benefit from the GIFT of righteousness that God gives through the blood of Christ and the cross in exchange for our own imperfect and flawed lives. But to think that God is content to allow people to go on wilfully defiling His holy temple totally sure that they are "covered" is a massive deception. The Scripture gives us plenty of warnings about that. We have to be sure that we are taking heed, placing God's Word in a higher place of respect than any Teacher.