Hi Kay, Good question.
It all comes down to what happens when you get forgiven, its much more than just having the slate wiped clean. Lets start from the beginning. When you become a Christian:
1. The Holy Spirit makes you into a new creation (2 Cor 5:17) and unites you to Christ. This means:
- all the benefits of Christ’s Sonship become yours as you become a co-heir (Rom 8:17), including eternal life.
- now you can address God as ‘Father’ (Rom 8:15, Gal 4:6).
2. Another of those benefits is Jesus’ righteousness, which is credited to you as a gift. In return, Jesus takes your guilt. (2 Cor 5:21, Rom 3:21-22).
3. Because of these things, you are forgiven and declared to be righteous before God (Rom 5:1).
So, you see, its not just forgiveness. The mechanism by which you are declared to be righteous before God is because you are united with Christ, meaning that he can bear the punishment you deserve, and you can be credited with his righteousness. The thing is, if you reject Jesus then you are doing more than saying ‘no’ to God’s forgiveness - something we might imagine God would forgive. You are actually rejecting the only means by which you can be saved (Hebrews 2:1-3). Jesus didn’t just die so that sinners could be forgiven. He died the death that sinners deserve, for those sinners who are united to him in that event (Rom 6:1-10), meaning that his resurrection is ours too if we are united to him. This is how you get eternal life.
I know this sounds a bit complicated, but its actually fairly simple. One famous theologian, Martin Luther, compared it to a marriage. When two people are married, you can’t punish one of them without punishing the other. If you put one in jail, the other still suffers. If you fine one, the other bears the financial cost as well. The relationship is too close to separate them. Similarly, if you reward one then the other is honored as well. Now marriage is an imperfect analogy, but it does help us to understand the picture. Christ’s death only ‘works’ for us if we are so close to him that his suffering becomes our suffering, his death our death, and his resurrection our resurrection. In this way he bears our guilt, and in this way we share his life. That means you can’t be forgiven for rejecting Jesus, because there is simply no way that that forgiveness could happen.
Hope that helps.