Summary: A brief look at the forgiveness of sins, past, present. and future
In order to render a response, it is vital to review the entirety of Scripture and not just verses extracted out of context.
Christians serve a God who is “rich in mercy” and continually proves His “great love for us.” He has made every Believer “alive with Christ even when” they “were dead in transgressions.” It is by His grace that they “have been saved.” There is absolutely nothing they could ever do to be worthy of it. The wondrous heavenly Father has “seated” them “with him in the heavenly realms in Christ Jesus” (Eph. 2:4-6).
Mankind is now dead in their sins because of Adam. But in Jesus, they are made alive! To be “alive in Him” means that they are dead in their transgressions because it is "by His grace we have been saved" (Eph 2:5). It is only by His grace and not by anything they could ever do. Since they “have been raised with” Jesus, their life is now “hidden with Christ in God.” They must now set their “hearts on things above, where Christ is seated at the right hand of God” (Col. 3:1-3).
When a Christian sins their position before God stays the same but their condition at that moment is changed. If a husband or wife hurts the other and does not apologize or repent of his or her mistake, their condition, or relationship, is harmed, even though they are still just as married as when they were in newly-wedded bliss. In our current legal system, the only way the behavior could change their position is if they both mutually agree to get a divorce.
"My dear children, I write this to you so that you will not sin. But if anybody does sin, we have one who speaks to the Father in our defense - Jesus Christ, the Righteous One." (1 John 2:1-2)
The intercession of Jesus assures them that they have forgiveness. "In the same way, the Spirit helps us in our weakness. We do not know what we ought to pray for, but the Spirit Himself intercedes for us with groans that words cannot express" (Rom 8:26).
When a Christian sins, Jesus is always interceding on their behalf before God the Father. Their relationship with Jesus is based upon what He has already done for them, not what they have done for Him.
“I give them ETERNAL life, and they shall NEVER perish; no one can snatch them out of my hand. My Father, who has given them to me, is greater than all; no one can snatch them out of my Father’s hand.” (John 10:28)
Ephesians 1:3–14 walks us through the redemption story. In the past of the Born Again Christian “He chose us in him before the foundation of the world, that we should be holy and blameless before him. In love, he predestined us for adoption as sons through Jesus Christ. (Eph vs. 4–5) and sealed us with the promised Holy Spirit.
In the 'present' of the Born-Again Christian, “we have redemption through his blood, the forgiveness of our trespasses, according to the riches of his grace, which he lavished upon us, in all wisdom and insight making known to us the mystery of his will, according to his purpose. (vs 1:7–9) If we are left in our sin, then we are separated from God. Only the single one time sacrifice of Jesus can bring the holy God so close that He never leaves.