We should definitely be wary of the wolf in sheep's clothing. The great pretender is Satan who tries to disguise himself as an angel of light, but is actually the darkness and depths of the pit.
There are many who do profess Jesus, but who do not posses Him and the Holy Spirit does not indwell them. They are those who pretend to be in the body of Christ, but who are people that Christ does not recognize. (This is not to say that these people will never know Christ, just that the wickedness they commit and stir up only produces rotten fruit.)
1 Corinthians 13 is the Christian plumb line. It is a description of agape kind of love which only comes from the Father and is gifted to us to pour out on others as we become more and more full of the Holy Spirit. Agape is active and is quite literally, love in action. Which is why Peter had such a hard time understanding what Jesus was asking in His question, "Peter, do you love Me?" Jesus was talking about full-blown love displayed by the action of feeding His sheep. Peter was insisting that He loved Jesus Himself, and Jesus wanted Peter's proof of love with actions.
Hebrew 3:12 Watch out, brothers, so that there won't be in any of you an evil, unbelieving heart that departs from the living God.
This is a warning to Christians to keep their congregations pure from the world. Because of several other verses... Hebrews 10:22 let us draw near with a true heart in full assurance of faith, our hearts sprinkled [clean] from an evil conscience and our bodies washed in pure water.
Where does the true heart come from? Only from God. Man cannot achieve it alone.
Psalm 17:3 You have tested my heart; You have visited by night; You have tried me and found nothing [evil]; I have determined that my mouth will not sin. [Or [evil]; my mouth will not sin] David claims God found nothing evil in his heart after searching it... yet, David did commit adultery. The lust in his heart for Bathsheba was not always there. David should have been with his army fighting that battle instead of reclining by the fire and eating bonbons. But there is one thing to cling to 1 Samuel 15:13 So Samuel took the horn of oil, anointed him in the presence of his brothers, and the Spirit of the LORD took control of David from that day forward. The black sins of murder and adultery were events in David's choices, but they do not define him. David was the apple of God's eye.
But David still sinned with Bathsheba. David still prayed for God to keep him safe from evil. Psalm 141:4 Do not let my heart turn to any evil thing or wickedly perform reckless acts with men who commit sin. Do not let me feast on their delicacies. It is only the foolish that live godlessly and the wicked that God hates. Prov 3:32 for the devious are detestable to the LORD, but He is a friend to the upright. Isaiah 32:6 6 For a fool speaks foolishness and his mind plots iniquity. He lives in a godless way and speaks falsely about the LORD. He leaves the hungry empty and deprives the thirsty of drink.
Another example of a warning to believers against unbelievers in their midst is the letter to Pergamum 14 But I have a few things against you. You have some there who hold to the teaching of Balaam, who taught Balak to place a stumbling block in front of the sons of Israel: to eat meat sacrificed to idols and to commit sexual immorality. 15 In the same way, you also have those who hold to the teaching of the Nicolaitans. 16 Therefore repent! Otherwise, I will come to you quickly and fight against them with the sword of My mouth. Them, not you. Interesting.
An example of warning to unbelievers is the letter to Laodicea. 16 So, because you are lukewarm, and neither hot nor cold, I am going to vomit you out of My mouth. I believe this is to unbelievers because according to Jesus and the writers of the New Testament, the Holy Spirit lives within us and He becomes one with us (John 17) and in doing so, God creates a completely new creature. 2 Cor 5:17 Therefore if anyone is in Christ, there is a new creation; old things have passed away, and look, new things [Other mss read look, all new things] have come. Galatians 6:15 For [ Other mss add in Christ Jesus] both circumcision and uncircumcision mean nothing; [what matters] instead is a new creation. To spew out of the mouth means that it never went to the stomach. It was intimate, but the relationship never developed into full agape love.
This letter also says that these people are naked, blind and poor even though they do not know it. Believers are not naked. Matt 22:11 And the king having come in to view those reclining, saw there a man not clothed with clothing of the marriage-feast, 2 Cor 5:3 if so be that, having clothed ourselves, we shall not be found naked. There are lots of verses that describe Believers as rich and as having Spiritual 20/20 vision and discernment.
Jesus says He will spew them out of His mouth. What we have here is a warning to the pretend Christians. Those who go to church and go through the motions of being Christian, who live a public life beyond reproach but their hearts are black with sin, never being circumcised and therefore insensitive to the Holy Spirit, even perhaps insensitive to their own conscience. They are absolutely miserable, and don't know why they are miserable. They think because they are wealthy according to the world's standards and go to church, they are saved.
It is these black hearts that we must be wary against because they live a make-believe righteousness that is sickening to God. These are the misguided ones and can cause quite a bit of strife amongst God's children.