What does it mean to say that believers are forgiven past, present, future sins?

It means that our sin will never be imputed to us. Our sins were taken away by Christ!

John 1 “²⁹ The next day, he saw Jesus coming to him, and said, “Behold, the Lamb of God, who takes away the sin of the world! “

While we were unbelievers, we were under law. At that time, sin was imputed to us because of the Law that demanded perfection.

But when we believed in Christ, our sins were fully imputed on Him. The Law was fulfilled and its demands met on the cross.

The believer is not under law and therefore our sins will never be imputed to us.

Romans 4 ” 5 But to him who doesn’t work, but believes in him who justifies the ungodly, his faith is accounted for righteousness. 6 Even as David also pronounces blessing on the man to whom God counts righteousness apart from works,7 “Blessed are they whose iniquities are forgiven, whose sins are covered.8 Blessed is the man whom the Lord will by NO MEANS charge with sin.”

Why is that? It’s because Christ died for our sins to make us perfect forever.

Hebrews 10 “10 by which will we have been sanctified through the offering of the body of Jesus Christ once for all.“I will remember their sins and their iniquities no more.”Now where forgiveness of these is, there is no more offering for sin.”

Salvation means that we are forgiven all sins. They will never be held to our account.

Christ took the judgment of our sins and we received the blessing of His righteousness.

In fact, Jesus became sin on the cross, and He was raised from the dead to make us righteous in Him.

2nd Corinthians 5 ” ¹⁷ Therefore if anyone is in Christ, he is a new creation. The old things have passed away. Behold, all things have become new. ¹⁸ But all things are of God, who reconciled us to himself through Jesus Christ, and gave to us the ministry of reconciliation; ¹⁹ namely, that God was in Christ reconciling the world to himself, not reckoning to them their trespasses, and having committed to us the word of reconciliation. ²⁰ We are therefore ambassadors on behalf of Christ, as though God were entreating by us: we beg you on behalf of Christ, be reconciled to God. ²¹ For him who knew no sin he made to be sin on our behalf; so that in him we might become the righteousness of God.

John 5 “²⁴ “Most certainly I tell you, he who hears my word and believes him who sent me has eternal life, and doesn’t come into judgment, but has passed out of death into life.”

God will NEVER judge the believer for sins because Christ made the believer righteous.