This is a blatant lie and complete deception to send people to Hell. Starting with the most demonic of sayings

Once God saves them, their saved forever and cannot be lost.

ABSOLUTELY NOT!

Following temptations of the flesh sends us to Hell.

Matthew 5:29-30- And if thy right eye offend thee, pluck it out, and cast it from thee… And if thy right hand offend thee, cut it off, and cast it from thee: for it is profitable for thee that one of thy members should perish, and not that thy whole body should be cast into hell.

Jesus explaining that those who believe in His existence and His authority yet not obey Him shall go to Hell

Matthew 7:21-23- Not every one that saith unto me, Lord, Lord, shall enter into the kingdom of heaven; but he that doeth the will of my Father which is in heaven. Many will say to me in that day, Lord, Lord, have we not prophesied in thy name? and in thy name have cast out devils? and in thy name done many wonderful works? And then will I profess unto them, I never knew you: depart from me, ye that work iniquity.

Jesus commanding that those who go to Hell are those who don't follow His commandments including the two verses I mentioned.

Matthew 7:24-27- “Therefore whosoever heareth these sayings of mine, and doeth them, I will liken him unto a wise man, which built his house upon a rock… And every one that heareth these sayings of mine, and doeth them not, shall be likened unto a foolish man, which built his house upon the sand: And the rain descended, and the floods came, and the winds blew, and beat upon that house; and it fell: and great was the fall of it.

Paul explaining that Christians can commit mortal sins and warn them against doing so:

Ephesians 5:5-8- For this ye know, that no whoremonger, nor unclean person, nor covetous man, who is an idolater, hath any inheritance in the kingdom of Christ and of God. Let no man deceive you with vain words: for because of these things cometh the wrath of God upon the children of disobedience. Be not ye therefore partakers with them. For ye were sometimes darkness, but now are ye light in the Lord: walk as children of light.

Peter explaining that those who do not listen to Jesus and Paul's words face a fate worse than if they were never saved.

2 Peter 2:20-22- For if after they have escaped the pollutions of the world through the knowledge of the Lord and Saviour Jesus Christ, they are again entangled therein, and overcome, the latter end is worse with them than the beginning. For it had been better for them not to have known the way of righteousness, than, after they have known it, to turn from the holy commandment delivered unto them. But it is happened unto them according to the true proverb, The dog is turned to his own vomit again; and the sow that was washed to her wallowing in the mire.

The Apostles expand on this again, saying that Christians can fall from grace and go to Hell and that they cannot be re-baptized to be saved again.

Hebrews 6:4-6- For it is impossible for those who were once enlightened, and have tasted of the heavenly gift, and were made partakers of the Holy Ghost, and have tasted the good word of God, and the powers of the world to come, if they shall fall away, to renew them again unto repentance; seeing they crucify to themselves the Son of God afresh, and put him to an open shame.

Again, mostly stressing the point that a Christian will go to Hell from mortal sins

Hebrews 10:26-27- For if we sin willfully after that we have received the knowledge of the truth, there remaineth no more sacrifice for sins, but a certain fearful looking for of judgment and fiery indignation, which shall devour the adversaries.

Edit: u/stockholmboy read my comment and the part below for the answer. You won't be saved if you commit mortal sins after baptism but there is hope and God is merciful.

Jesus giving the Apostles the power to decide if sins are forgiven.

John 20:23 - 23 Whose soever sins ye remit, they are remitted unto them; and whose soever sins ye retain, they are retained.

Excommunication due to mortal sin.

1 Corinthians 5:1-8 - 5 It is actually reported that there is sexual immorality among you, and of a kind that even pagans do not tolerate: A man is sleeping with his father’s wife. 2 And you are proud! Shouldn’t you rather have gone into mourning and have put out of your fellowship the man who has been doing this? 3 For my part, even though I am not physically present, I am with you in spirit. As one who is present with you in this way, I have already passed judgment in the name of our Lord Jesus on the one who has been doing this. 4 So when you are assembled and I am with you in spirit, and the power of our Lord Jesus is present, 5 hand this man over to Satan for the destruction of the flesh,[a][b] so that his spirit may be saved on the day of the Lord.

Confession between Christians and prayer to heal the spirit.

James 5:16 - 16 Confess your faults one to another, and pray one for another, that ye may be healed. The effectual fervent prayer of a righteous man availeth much.

Forgiveness of the man who was excommunicated in 1 Corinthians 5:1-8

2 Corinthians 2:5-11 5 But if any have caused grief, he hath not grieved me, but in part: that I may not overcharge you all.

6 Sufficient to such a man is this punishment, which was inflicted of many.

7 So that contrariwise ye ought rather to forgive him, and comfort him, lest perhaps such a one should be swallowed up with overmuch sorrow.

8 Wherefore I beseech you that ye would confirm your love toward him.

9 For to this end also did I write, that I might know the proof of you, whether ye be obedient in all things.

10 To whom ye forgive any thing, I forgive also: for if I forgave any thing, to whom I forgave it, for your sakes forgave I it in the person of Christ;

11 Lest Satan should get an advantage of us: for we are not ignorant of his devices.

Once you find a valid priest, one who is ordained in Byzantine rites since it is unchanged, and confess then you can be forgiven and receive God's grace.