Beware lest there be among you a root bearing poisonous and bitter fruit, one who, when he hears the words of this sworn covenant, blesses himself in his heart, saying, ‘I shall be safe, though I walk in the stubbornness of my heart.’ Deuteronomy 29:18b-19a

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full passage:

Beware lest there be among you a man or woman or clan or tribe whose heart is turning away today from the LORD our God to go and serve the gods of those nations. Beware lest there be among you a root bearing poisonous and bitter fruit, one who, when he hears the words of this sworn covenant, blesses himself in his heart, saying, ‘I shall be safe, though I walk in the stubbornness of my heart.’ This will lead to the sweeping away of moist and dry alike. The LORD will not be willing to forgive him, but rather the anger of the LORD and his jealousy will smoke against that man, and the curses written in this book will settle upon him, and the LORD will blot out his name from under heaven. And the LORD will single him out from all the tribes of Israel for calamity, in accordance with all the curses of the covenant written in this Book of the Law. Deuteronomy 29:18-21

related:

For the moment all discipline seems painful rather than pleasant, but later it yields the peaceful fruit of righteousness to those who have been trained by it. Therefore lift your drooping hands and strengthen your weak knees, and make straight paths for your feet, so that what is lame may not be put out of joint but rather be healed. trive for peace with everyone, and for the holiness without which no one will see the Lord. See to it that no one fails to obtain the grace of God; that no “root of bitterness” springs up and causes trouble, and by it many become defiled; that no one is sexually immoral or unholy like Esau, who sold his birthright for a single meal. For you know that afterward, when he desired to inherit the blessing, he was rejected, for he found no chance to repent, though he sought it with tears. Hebrews 12:11-17 (Also see Heb. 12:12-29, Titus 2:11-3:8.)

For it is impossible, in the case of those who have once been enlightened, who have tasted the heavenly gift, and have shared in the Holy Spirit, and have tasted the goodness of the word of God and the powers of the age to come, and then have fallen away, to restore them again to repentance, since they are crucifying once again the Son of God to their own harm and holding him up to contempt. For land that has drunk the rain that often falls on it, and produces a crop useful to those for whose sake it is cultivated, receives a blessing from God. But if it bears thorns and thistles, it is worthless and near to being cursed, and its end is to be burned. Though we speak in this way, yet in your case, beloved, we feel sure of better things—things that belong to salvation. For God is not unjust so as to overlook your work and the love that you have shown for his name in serving the saints, as you still do. And we desire each one of you to show the same earnestness to have the full assurance of hope until the end, so that you may not be sluggish, but imitators of those who through faith and patience inherit the promises. Hebrews 6:4-12

These are waterless springs and mists driven by a storm. For them the gloom of utter darkness has been reserved. For, speaking loud boasts of folly, they entice by sensual passions of the flesh those who are barely escaping from those who live in error. They promise them freedom, but they themselves are slaves of corruption. For whatever overcomes a person, to that he is enslaved. For if, after they have escaped the defilements of the world through the knowledge of our Lord and Savior Jesus Christ, they are again entangled in them and overcome, the last state has become worse for them than the first. For it would have been better for them never to have known the way of righteousness than after knowing it to turn back from the holy commandment delivered to them. What the true proverb says has happened to them: “The dog returns to its own vomit, and the sow, after washing herself, returns to wallow in the mire.” 2 Peter 2:17-22

Do not be deceived: “Bad company ruins good morals.” Wake up from your drunken stupor, as is right, and do not go on sinning. For some have no knowledge of God. I say this to your shame. 1 Corinthians 15:33-34 (Also see 1Cor. 3:16-18, 6:9-20, 9:24-10:13, 2Cor. 6:14-7:1, 11:2-4, Gal. 6:7-8, Eph. 5:5-6, 2Tim. 3:12-17, Jam. 1:12-18, 1John 3:7-10, Mark 7:20-23.)

Again he began to teach beside the sea. And a very large crowd gathered about him, so that he got into a boat and sat in it on the sea, and the whole crowd was beside the sea on the land. And he was teaching them many things in parables, and in his teaching he said to them: “Listen! Behold, a sower went out to sow. And as he sowed, some seed fell along the path, and the birds came and devoured it. Other seed fell on rocky ground, where it did not have much soil, and immediately it sprang up, since it had no depth of soil. And when the sun rose, it was scorched, and since it had no root, it withered away. Other seed fell among thorns, and the thorns grew up and choked it, and it yielded no grain. And other seeds fell into good soil and produced grain, growing up and increasing and yielding thirtyfold and sixtyfold and a hundredfold.” And he said, “He who has ears to hear, let him hear.” And when he was alone, those around him with the twelve asked him about the parables. And he said to them, “To you has been given the secret of the kingdom of God, but for those outside everything is in parables, so that “they may indeed see but not perceive, and may indeed hear but not understand, lest they should turn and be forgiven.” And he said to them, “Do you not understand this parable? How then will you understand all the parables? The sower sows the word. And these are the ones along the path, where the word is sown: when they hear, Satan immediately comes and takes away the word that is sown in them. And these are the ones sown on rocky ground: the ones who, when they hear the word, immediately receive it with joy. And they have no root in themselves, but endure for a while; then, when tribulation or persecution arises on account of the word, immediately they fall away. And others are the ones sown among thorns. They are those who hear the word, but the cares of the world and the deceitfulness of riches and the desires for other things enter in and choke the word, and it proves unfruitful. But those that were sown on the good soil are the ones who hear the word and accept it and bear fruit, thirtyfold and sixtyfold and a hundredfold.” And he said to them, “Is a lamp brought in to be put under a basket, or under a bed, and not on a stand? For nothing is hidden except to be made manifest; nor is anything secret except to come to light. If anyone has ears to hear, let him hear.” And he said to them, “Pay attention to what you hear: with the measure you use, it will be measured to you, and still more will be added to you. For to the one who has, more will be given, and from the one who has not, even what he has will be taken away.” Mark 4:1-25

Then he left the crowds and went into the house. And his disciples came to him, saying, “Explain to us the parable of the weeds of the field.” He answered, “The one who sows the good seed is the Son of Man. The field is the world, and the good seed is the sons of the kingdom. The weeds are the sons of the evil one, and the enemy who sowed them is the devil. The harvest is the end of the age, and the reapers are angels. Just as the weeds are gathered and burned with fire, so will it be at the end of the age. The Son of Man will send his angels, and they will gather out of his kingdom all causes of sin and all law-breakers, and throw them into the fiery furnace. In that place there will be weeping and gnashing of teeth. Then the righteous will shine like the sun in the kingdom of their Father. He who has ears, let him hear. Matthew 13:36-43

“Again, the kingdom of heaven is like a net that was thrown into the sea and gathered fish of every kind. When it was full, men drew it ashore and sat down and sorted the good into containers but threw away the bad. So it will be at the end of the age. The angels will come out and separate the evil from the righteous and throw them into the fiery furnace. In that place there will be weeping and gnashing of teeth. Matthew 13:47-50

“I am the true vine, and my Father is the vinedresser. Every branch in me that does not bear fruit he takes away, and every branch that does bear fruit he prunes, that it may bear more fruit. Already you are clean because of the word that I have spoken to you. Abide in me, and I in you. As the branch cannot bear fruit by itself, unless it abides in the vine, neither can you, unless you abide in me. I am the vine; you are the branches. Whoever abides in me and I in him, he it is that bears much fruit, for apart from me you can do nothing. If anyone does not abide in me he is thrown away like a branch and withers; and the branches are gathered, thrown into the fire, and burned. If you abide in me, and my words abide in you, ask whatever you wish, and it will be done for you. By this my Father is glorified, that you bear much fruit and so prove to be my disciples. As the Father has loved me, so have I loved you. Abide in my love. If you keep my commandments, you will abide in my love, just as I have kept my Father’s commandments and abide in his love. These things I have spoken to you, that my joy may be in you, and that your joy may be full. “This is my commandment, that you love one another as I have loved you. Greater love has no one than this, that someone lay down his life for his friends. You are my friends if you do what I command you. No longer do I call you servants, for the servant does not know what his master is doing; but I have called you friends, for all that I have heard from my Father I have made known to you. You did not choose me, but I chose you and appointed you that you should go and bear fruit and that your fruit should abide, so that whatever you ask the Father in my name, he may give it to you. These things I command you, so that you will love one another. “If the world hates you, know that it has hated me before it hated you. If you were of the world, the world would love you as its own; but because you are not of the world, but I chose you out of the world, therefore the world hates you. Remember the word that I said to you: ‘A servant is not greater than his master.’ If they persecuted me, they will also persecute you. If they kept my word, they will also keep yours. But all these things they will do to you on account of my name, because they do not know him who sent me. John 15:1-21

But if some of the branches were broken off, and you, although a wild olive shoot, were grafted in among the others and now share in the nourishing root of the olive tree, do not be arrogant toward the branches. If you are, remember it is not you who support the root, but the root that supports you. Then you will say, “Branches were broken off so that I might be grafted in.” That is true. They were broken off because of their unbelief, but you stand fast through faith. So do not become proud, but fear. For if God did not spare the natural branches, neither will he spare you. Note then the kindness and the severity of God: severity toward those who have fallen, but God’s kindness to you, provided you continue in his kindness. Otherwise you too will be cut off. And even they, if they do not continue in their unbelief, will be grafted in, for God has the power to graft them in again. For if you were cut from what is by nature a wild olive tree, and grafted, contrary to nature, into a cultivated olive tree, how much more will these, the natural branches, be grafted back into their own olive tree. Lest you be wise in your own sight, I do not want you to be unaware of this mystery, brothers: a partial hardening has come upon Israel, until the fullness of the Gentiles has come in. Romans 11:17-25