My Bible notes that false teachers often camouflage their lies with truth, making it difficult to catch their errors. I think we are seeing this more and more today. Some religions claiming to be Christian are using common language of Christianity but underneath are teaching far from Biblical truths.
Sadly, many Christian churches are watering down the truth. They are not preaching sin and not preaching the consequences of sin. We see the New Age religion that teaches freedom in believing what is "right" for you. This belief is filtering into many denominations causing church and denominational splits.
How do we protect ourselves from false teachers? We must have a firm foundation. We need to walk daily with God and daily in his word. Being saved will get us to heaven, but we also need protection from the evil one who wars against us every day.
If we are not firmly planted in God's truths, we can be easily mislead by the false teachers of this world. Knowing His word and praying for wisdom will help us be prepared to recognize the lies presented to us even when camouflaged by familiar teachings.
But there were also false prophets among the people, just as there will be false teachers among you. They will secretly introduce destructive destructive heresies, even denying the sovereign Lord who bought them - bringing swift destruction on themselves. Many will follow their shameful ways and will bring the way of truth into disrepute. 2 Peter 2:1-2