1 John 5:16-17 states that:
"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. All wrongdoing is sin, and there is sin that does not lead to death."
A sin that "leads to death" is a mortal sin. Mortal sin is a grave or serious offense against God's Law. It destroys grace in the soul and places you in a state of separateness or disconnection from God (because God is all-good and all-holy, thus we cannot abide in Him or be close to Him if we are in a state of sin).
Mortal sin is basically spiritual death, and if one dies in such a state, he/she goes to Hell. With that said, God is always waiting for us to repent (hence why we believe in the Sacrament of Penance aka Confession). God is always willing to forgive us our sins and restore us to a state of sanctifying grace, but repentance cannot be done after death. So up until your last breath, you always have the opportunity to repent and receive God's forgiveness and grace. So, I will agree with
@momi in the sense that we cannot be taken away from God or separated from Him if we are truly seeking Him and repentance. WE may fail, because we are weak, but God will not fail us. However, God will not force us to go to Heaven, and He will not force His grace on us. We must accept it and cooperate with Him. This is why Christians talk about having a "relationship" with God, with Jesus Christ, because we acknowledge that He is the "Lover" and we are the "Beloved," and He seeks for us to love Him and say "Yes" to Him.