Read the Parable of the Wise and Foolish Virgins, the Parable of the Talents, the Parable of the Prodigal Son, what the Lord mentions about those who commit Blasphemy against the Holy Spirit, and what the Lord says about those who take the mark of the beast.
Using the mark of the beast as an example. If you are a truly born against spirit who is saved in Jesus Christ, according to the Lord's own words, you will be eternally damned and will not be allowed to repent and be saved again.
If you commit blasphemy against the Holy Spirit, you can never be forgiven.
And if you do not do the work the Lord assigns you in His house, you will lose your salvation as shown in the Parable of the Talents.
If you receive the annointing of the Holy Spirit, and don't keep the annointing topped up as the wise virgins did, but let it run out as the foolish virgins did, then you will be locked out of Heaven.
But there are times when you as a saved person can fall into temptation and sin, and you can still repent and be saved. Consider when Jesus was denied by Peter before the crucifixion, and after the resurrection, how Jesus asked him 3 times to feed His sheep!
You have to know the context of the sin and the conditions attached.
When you are born, you are born spiritually dead belonging to the devil and destined for Hell.
Once you are born again, you can sin and be forgiven depending upon what the sin is. But if you sin the sin that kills your spirit, then you can never be reborn into life again and will remain among the damned.
1 John 5:16
New International Version
16 If you see any brother or sister commit a sin that does not lead to death, you should pray and God will give them life. I refer to those whose sin does not lead to death. There is a sin that leads to death. I am not saying that you should pray about that.
But when you invite Jesus into your life and accept Him as your Lord and Saviour, then at that point you have entered into eternal life because you become spiritually born of Jesus. At that point you are expected to repent and turn your life around. And once you are saved, you are expected to know God's will for your life and do it. See, Matthew 7:21-23.