Unrepented mortal sin requires eternal hellfire but unrepented venial sin does not.
"Mortal" sins and "not mortal" sins (aka venial sins) per 1 John 5:16-17:
If any one sees his brother committing what is not a mortal sin, he will ask, and God will give him life for those whose sin is not mortal. There is sin which is mortal; I do not say that one is to pray for that. 17 All wrongdoing is sin, but there is sin which is not mortal. RSV
Unrepented and unconfessed venial sins make it easier for a person to later fall away from his faith because his conscience gets negligent and therefore he can be more easily tempted to commit a mortal sin.
Just one mortal sin causes the loss of the Holy Spirit's sanctifying presence in a person's soul. If this person does not repent and confess this sin before he dies, he will not be approved to enter/inherit eternal life. It is the Holy Spirit's abiding presence in a person's soul at the time of his death which guarantees that he will be approved to inherit/enter into eternal life. 2 Corinthians 1:21-22 If the Holy Spirit is not present in your soul at the time of your death, Jesus will claim that He does not know you. Luke 13:24-28 He will claim He never knew you. Matthew 7:22-23
1 John 5:16-17 If anyone sees his brother sinning a sin which does not lead to death, he will ask, and He will give him life for those who commit sin not leading to death. There is sin leading to death. I do not say that he should pray about that. 17 All unrighteousness is sin, and there is sin not leading to death. NKJV
"Sin leading to death" is unconfessed and unrepented "mortal sin". If the person does not confess and repent the mortal sin before his death, he will receive condemnation ("a death sentence" meaning he loses eternal life) instead of inheriting eternal life with God.
Jesus gives us an example of venial sin which will require a "cleansing" fire also called a "purging fire" after death which is the punishment for unrepented venial sins
1 Corinthians 3:12-15,
Hebrews 12:29 and Jesus also gives us an example of mortal sin which will require hellfire after death which is the required punishment for unrepented mortal sins:
Matthew 5:21-22 You have heard that it was said to the men of old, ‘You shall not kill; and whoever kills shall be liable to judgment.’ 22 But I say to you that every one who is angry with his brother shall be liable to judgment; whoever insults his brother shall be liable to the council, and whoever says, ‘You fool!’ shall be liable to the hell of fire.
"Liable to the council" refers to the Sanhedrin Council which meted out punishment such as prison time for people who were convicted for breaking Jewish laws. Jesus uses this as a metaphor for a person's punishment in the purging/cleansing fire after death.
Anger at a brother is considered neutral unless this anger escalates into insulting his brother. Insulting his brother is a venial sin. Anger at a brother which escalates into contempt for his brother is a mortal sin. Every person is created in the image and likeness of God
Genesis 1:26-27 and therefore showing contempt for any one of God's human creatures is a grievous sin.
Matthew 5:43-45
Jesus gives us another metaphor regarding this purging/cleansing fire as a prison:
Matthew 5:23-26 So if you are offering your gift at the altar, and there remember that your brother has something against you, 24 leave your gift there before the altar and go; first be reconciled to your brother, and then come and offer your gift. 25 Make friends quickly with your accuser, while you are going with him to court, lest your accuser hand you over to the judge, and the judge to the guard, and you be put in prison; 26 truly, I say to you, you will never get out till you have paid the last penny.
Unrepented mortal sin requires eternal hellfire but unrepented venial sin does not.