Because it teaches that a person can sin as to be eternally lost.

Now the serpent was more cunning than any beast of the field which the Lord God had made. And he said to the woman, "Has God indeed said, 'You shall not eat of every tree of the garden '?" And the woman said to the serpent, "We may eat the fruit of the trees of the garden; but of the fruit of the tree which is in the midst of the garden, God has said, 'You shall not eat it, nor shall you touch it, lest you die. ‘ " Then the serpent said to the woman, "You will not surely die.

Therefore let him who thinks he stands take heed lest he fall. -1 Corinthians 10:12 You have become estranged from Christ, you who attempt to be justified by law; you have fallen from grace. - Galatians 5:4

For if, after they have escaped the pollutions of the world through the knowledge of the Lord and Savior Jesus Christ, they are again entangled in them and overcome, the latter end is worse for them than the beginning. For it would have been better for them not to have known the way of righteousness, than having known it, to turn from the holy commandment delivered to them.

But it has happened to them according to the true proverb: "A dog returns to his own vomit," and, "a sow, having washed, to her wallowing in the mire.“ - 2 Peter 2:20-22

Now the Spirit expressly says that in latter times some will depart from the faith, giving heed to deceiving spirits and doctrines of demons, speaking lies in hypocrisy, having their own conscience seared with a hot iron, forbidding to marry, and commanding to abstain from foods which God created to be received with thanksgiving by those who believe and know the truth. - 1 Timothy 4:1-3

Brethren, if anyone among you wanders from the truth, and someone turns him back, let him know that he who turns a sinner from the error of his way will save a soul from death and cover a multitude of sins. - James 5:19-20

1.Demas

Luke the beloved physician and Demas greet you. - Colossians 4:14 as do Mark, Aristarchus, Demas, Luke, my fellow laborers. - Philemon 1:24

for Demas has forsaken me, having loved this present world, and has departed for Thessalonica — Crescens for Galatia, Titus for Dalmatia. - 1 Timothy 4:10

1.Demas 2.Followers of Christ (John 6:64-67)

But there are some of you who do not believe." For Jesus knew from the beginning who they were who did not believe, and who would betray Him. And He said, " Therefore I have said to you that no one can come to Me unless it has been granted to him by My Father.“

From that time many of His disciples went back and walked with Him no more. Then Jesus said to the twelve, " Do you also want to go away?“ -John 6:64-67

1.Demas 2.Followers of Christ (John 6:64-67) 3.Ananias & Sapphira

But a certain man named Ananias, with Sapphira his wife, sold a possession. And he kept back part of the proceeds, his wife also being aware of it, and brought a certain part and laid it at the apostles ' feet. But Peter said, " Ananias, why has Satan filled your heart to lie to the Holy Spirit and keep back part of the price of the land for yourself? While it remained, was it not your own? And after it was sold, was it not in your own control? Why have you conceived this thing in your heart? You have not lied to men but to God."

Then Ananias, hearing these words, fell down and breathed his last. So great fear came upon all those who heard these things. And the young men arose and wrapped him up, carried him out, and buried him. Now it was about three hours later when his wife came in, not knowing what had happened. And Peter answered her, " Tell me whether you sold the land for so much?“ She said, " Yes, for so much.“

Then Peter said to her, " How is it that you have agreed together to test the Spirit of the Lord? Look, the feet of those who have buried your husband are at the door, and they will carry you out." Then immediately she fell down at his feet and breathed her last. And the young men came in and found her dead, and carrying her out, buried her by her husband. - Acts 5:1-10

Now the works of the flesh are evident, which are: adultery, fornication, uncleanness, lewdness, idolatry, sorcery, hatred, contentions, jealousies, outbursts of wrath, selfish ambitions, dissensions, heresies, envy, murders, drunkenness, revelries, and the like; of which I tell you beforehand, just as I also told you in time past, that those who practice such things will not inherit the kingdom of God. - Galatians 5:19-21

1. False Hope

For when they speak great swelling words of emptiness, they allure through the lusts of the flesh, through lewdness, the ones who have actually escaped from those who live in error. While they promise them liberty, they themselves are slaves of corruption; for by whom a person is overcome, by him also he is brought into bondage.

For if, after they have escaped the pollutions of the world through the knowledge of the Lord and Savior Jesus Christ, they are again entangled in them and overcome, the latter end is worse for them than the beginning. -2 Peter 2:18-20

I tell you, no; but unless you repent you will all likewise perish. – Luke 13:3,5

1. False Hope 2.False teaching separates souls from God.

You therefore, beloved, since you know this beforehand, beware lest you also fall from your own steadfastness, being led away with the error of the wicked; - 2 Peter 3:17

Remember therefore from where you have fallen; repent and do the first works, or else I will come to you quickly and remove your lampstand from its place — unless you repent. - Revelation 2:5

You have been warned! Therefore let him who thinks he stands take heed lest he fall. - 1 Corinthians 10:12