Already Forgiven: Here are some verses that show clearly that the sins of all people were forgiven and forgotten on the Cross of Christ. The widely held belief that you must "accept by faith the gift of forgiveness" is simply not found in the Bible; it is the gift of righteousness that you must accept by faith (Romans 5:17). All are forgiven, but not all are saved.
(At the bottom of this page are verses about the eternal sin of unbelief, a.k.a. "blasphemy against the Holy Spirit.")
2 Corinthians 5:19 namely, that God was in Christ reconciling the world to Himself, not counting their trespasses against them, and He has committed to us the word of reconciliation.
Hebrews 8:12 For I will be merciful to their iniquities, and I will remember their sins no more.
Colossians 1:20 and through Him to reconcile all things to Himself, having made peace through the blood of His cross; through Him, I say, whether things on earth or things in heaven.
Mark 3:28 Truly I say to you, all sins shall be forgiven the sons of men
Hebrews 10:12 but He, having offered one sacrifice for sins for all time, sat down at the right hand of God.
Romans 5:10a For if while we were enemies, we were reconciled to God through the death of His Son, ... (Notice this verse says we were reconciled while we were enemies, not after we become friends with God.)
Hebrews 2:9b ... Jesus, because of the suffering of death crowned with glory and honor, that by the grace of God He might taste death for everyone.
Hebrews 9:26b ... but now once at the consummation of the ages He (Jesus) has been manifested to put away sin by the sacrifice of Himself.
John 1:29 The next day he (John the Baptist) saw Jesus coming to him and said, "Behold, the Lamb of God who takes away the sin of the world!"
I John 2:2 and He Himself is the propitiation for our sins; and not for ours only, but also for those of the whole world.
The Unforgivable Sin: Blasphemy Against the Holy Spirit
John 16:8-9a "And He (Holy Spirit), when He comes, will convict the world concerning sin, and righteousness, and judgment;  concerning sin, because they do not believe in Me; ...
Note: the Holy Spirits convicts the world of its unbelief; He does not convict believers of their individual sins since God has already forgotten your sins (see Hebrews 8:12 above). It is your own conscience that causes you to feel guilt and remorse.
Matthew 12:31-32 Therefore I say to you, any sin and blasphemy shall be forgiven men, but blasphemy against the Spirit shall not be forgiven.  And whoever shall speak a word against the Son of Man, it shall be forgiven him; but whoever shall speak against the Holy Spirit, it shall not be forgiven him, either in this age, or in the age to come.
Mark 3: 28-29 Truly I say to you, all sins shall be forgiven the sons of men, and whatever blasphemies they utter;  but whoever blasphemes against the Holy Spirit never has forgiveness, but is guilty of an eternal sin.
Luke 12:10 "And everyone who speaks a word against the Son of Man, it will be forgiven him; but he who blasphemes against the Holy Spirit, it will not be forgiven him.