Mortal sin is a sin that leads to death. The Bible sometimes talks about sin that leads to death but does not explain what it is. Many people believe that mortal sin causes spiritual death, but some people think it talks about physical death.
The Bible says that all sin is mortal ( Romans 6:23 ). Sin generates physical death and spiritual death (separation from God). But whoever accepts Jesus as a savior resurrects spiritually (returns and has fellowship with God) and one day will resurrect physically, to live in Heaven.
Mortal sin according to the Catholic Church
“Mortal sin” is primarily a concept of the Catholic Church. According to Catholicism, mortal sin is a serious sin committed by a person who understands what he is doing and does it willingly (because he wants to commit that sin). Even if he has already accepted Jesus as a savior, the person who commits mortal sin is separated from God and goes to hell if he does not repent.
The Bible says that all sin is grave and leads to death and hell ( James 2:10 ). But God forgives all the sins of those who are saved. There is no need to be afraid of losing your salvation. But the saved person who commits sin suffers the consequences during his life. Therefore, it is important to ask for forgiveness for sins and to strive not to sin anymore.
See here: can a Christian lose his salvation?
Can a believer commit mortal sin?
The Bible does not say whether a believer can commit mortal sin. It also depends on the definition of mortal sin. The main verse that talks about sin leading to death is 1 John 5:16 and there is much debate about what it means:
Eternal spiritual death
Spiritual death is the eternal separation from God. Those who die without having their sins forgiven will be forever separated from God. If that is what the verse means, whoever is saved does not commit mortal sin, because the Bible says that nothing can separate us from God ( 1 John 2: 1-2 ).
The Bible says that all sin has forgiveness, except blasphemy against the Holy Spirit ( Mark 3: 28-29 ). If the sin that leads to death is a sin that will not be forgiven (guaranteeing hell), it is probably the same as blasphemy against the Holy Spirit. Whoever commits this sin does not repent, so he cannot be forgiven.
See also: what is blasphemy against the Holy Spirit?
Sin always has consequences for our lives. The punishment for some sins is physical death. This happened to Ananias and Sapphira, in the time of the apostles ( Acts 5: 3-5 ). Some other passages in the Bible also seem to indicate that physical death is the consequence of some sins.
If mortal sin is a sin that is punished with physical death, it may be possible for the believer to commit. God disciplines us, for our own good ( 1 Corinthians 11:32 ). Perhaps, in some cases when someone commits a serious sin without deviating, God decides that it is better to take their life. But the Bible does not say what kind of sin deserves that kind of punishment.
No argument is perfect. We may not be sure what mortal sin is, but the Bible teaches us that God loves us and forgives us when we repent . Just ask for forgiveness when we understand that we have committed sin. We need not be afraid.