“Where does the Bible say that our sins are forgiven past, present and future?”
Speaking of the Lord Jesus Christ, Hebrews 9:25,26 says,
“Nor yet that He should offer Himself often, as the high priest entereth into the holy place every year with blood of others;
“For then must He often have suffered since the foundation of the world: but now once in the end of the world hath He appeared to put away sin by the sacrifice of Himself.”
If you are saved, your sins were forgiven by the sacrifice of Christ. If He only died for your past sins, He would have to die for every new sin that you sin. He would have had to begin with Adam’s sins at “the foundation of the world” and never stopped suffering for our sins. Instead, He “put away sin by the sacrifice of Himself” (Heb. 9:26), “for this He did once, when He offered up Himself” (Heb. 7:27).
Of course, you “receive forgiveness of sins” (Acts 26:18) when you believe the gospel. But you don’t have to believe the gospel again every time you sin. If you are saved and forgiven of your sins, then no matter what sin you commit today, you can open your Bible tomorrow and Colossians 2:13 still says, “having forgiven you all trespasses,” past tense. The words on the page don’t change despite how often you grieve God’s Spirit with your sin. Forgiveness for the believer is always a done deal, a fait accompli.
But knowing that you “have…the forgiveness of sins” (Eph. 1:7) and are “sealed with that holy Spirit of promise” (Eph. 1:13), Paul says to “grieve not the holy Spirit of God whereby ye are sealed” (Eph. 4:30). And the thing that grieves a “holy” Spirit is sin. So if you “have…the forgiveness of sins, according to the riches of His grace” (Eph. 1:7), then don’t “continue in sin, that grace may abound” (Rom. 6:1). source