“Once we are saved, we are always saved. We don’t need to worry about mortal sins, because when we are justified, God forgives all of our sins—past, present, and future.”
Scripture teaches the opposite.
It is, of course, true that when we come to God and are justified that he forgives all of our past sins. However, this is not true of present and future sins.
One of the requirements for coming to God is repentance. Jesus’ own message was, “Repent and believe in the gospel” (Mark 1:15). Repentance involves an actual turning away from sin (see Day 53).
This means that if we are still willfully committing the kind of sins we know will separate us from God—mortal sins—then we have not repented and will not be justified. Justification therefore does not involve the remission of present, unrepented mortal sins.
Neither does it involve the forgiveness of sins that have not yet been committed. This is also something our Lord teaches. In fact, he teaches it in the model Christian prayer he gave us—the Lord’s Prayer—in which he taught us to pray: “Forgive us our debts, as we also have forgiven our debtors” (Matt. 6:12; cf. Luke 11:4).
Jesus even singles out this petition for special comment, underscoring its importance and stating: “For if you forgive men their trespasses, your heavenly Father also will forgive you; but if you do not forgive men their trespasses, neither will your Father forgive your trespasses” (Matt. 6:14–15).
Since we are meant to pray the Lord’s Prayer on an ongoing basis, Jesus teaches us to pray for forgiveness on an ongoing basis. Therefore, we need this forgiveness: As we commit new sins, we need to repent and be forgiven for them.
The claim that when we first come to God, he forgives all our sins “past, present, and future” may be pithy, but it is not what Scripture teaches. This is clear from many passages besides the Lord’s Prayer. The fact that we haven’t been forgiven for sins we haven’t yet committed is indicated by the New Testament’s warnings against mortal sin and its implications for our salvation, which would otherwise be meaningless (see Day 302).