There is much confusion these days concerning the forgiveness of sins. Some go as far as to say that our sin is only against ourselves, rather than them being primarily committed against God, as seen in Scripture (Gen 39:9; Ps 51:4; 1Jn 1:9). 
Also, in spite of numerous passages indicating that repentance and faith are necessary in order to receive forgiveness, such as Acts 10:43, which says, “whoever believes in Him will receive remission of sins,” it is becoming increasingly common to hear someone say that it is not necessary for one to believe in order to receive forgiveness of sins, since everyone was already forgiven at the cross 2,000 years ago.
It is true that Jesus, as the spotless Lamb of God, took away the sins of the world (Jn 1:29; 1Jn 3:5). His shed blood not only made propitiation to God for our sins, but for the sins of the whole world (kosmos) (Rom 3:25; 1Jn 2:2). By His once and for all sacrifice He purged our sins and then sat down at the right hand of the Father, having obtained eternal redemption for us (Heb 1:3; 9:12, 26-28). At the cross God the Father was in Christ, reconciling the world (kosmos) unto Himself, rather than holding our trespasses against us (2Cor 5:19). All created by Christ, whether in heaven or on earth, visible or invisible, were reconciled to the Father through the blood of His cross (Col 1:16,21).
The Scriptures clearly teach that Christ’s once for all substitutionary sacrifice made atonement for the sins of the whole world and not merely for the elect, or those who believe in this age. If one were to draw a conclusion from these verses alone in isolation from the overall teaching of Scripture concerning the forgiveness of sins, they could conclude that everyone since the cross is born into the world automatically forgiven for all their sins, without any need of repentance and faith on their part.
However, there is an important distinction in Scripture between God reconciling all unto Himself and all becoming reconciled unto Him (2Cor 5:19-20) – between the provision of forgiveness and our reception or appropriation of it (Acts 10:43).
What many fail to take into consideration is that the forgiveness obtained for us at the cross is not merely a relational forgiveness, such as when we forgive those who have offended us personally. Rather it is a judicial pardon or acquittal for transgressions against God’s immutable justice as the Judge of the living and the dead. That is why one must repent and believe on the Lord Jesus Christ in order to receive the forgiveness or pardon obtained for us at the cross. Justification and forgiveness before God as our Judge is not imputed to us apart from faith in Christ, our Passover, who was sacrificed on our behalf as our substitute (1Cor 5:7; Rom 4:24-25). It is an absolution granted to us in Christ by the Judge of all the earth.
It is for this reason that we see throughout the New Testament that repentance and faith must precede forgiveness and justification. After Jesus had risen from the dead, and moments before ascending to the Father, He commissioned His disciples saying, “that repentance and remission of sins should be preached in His name to all nations, beginning at Jerusalem” (Luke 24:47). That the Apostles understood Jesus to mean that repentance and faith were a prerequisite to receiving the remission of sins is made clear by Peter a few days later when he said:
“Repent therefore and be converted, that your sins may be blotted out, so that times of refreshing may come from the presence of the Lord.” (Acts 3:19)
Paul likewise proclaimed forgiveness of sins and justification as being conditioned upon faith, saying:
“Therefore let it be known to you, brethren, that through this Man is preached to you the forgiveness of sins; 39 and by Him everyone who believes is justified from all things from which you could not be justified by the law of Moses.” (Acts 13:38-39)
In Acts 26 Paul recounts to king Agrippa the Lord’s commission to him when He appeared to him on the road to Damascus. Here he gives more detail as to the Lord’s commission for him to the Gentiles. Jesus said to him:
“I now send you to open their eyes, in order to turn them from darkness to light, and from the power of Satan to God, that they may receive forgiveness of sins and an inheritance among those who are sanctified by faith in Me.” (Acts 26:17-18)
Here we see that the message Paul was to proclaim to the Gentiles was that they were to repent, turning from darkness to light in order that they may receive forgiveness of sins, being sanctified by faith in Christ. Apart from such repentance and faith we have no forgiveness nor inheritance among those who have been sanctified by faith in Him.
So we see that, contrary to the claim of some that everyone was already forgiven at the cross, Scriptures teach that forgiveness is granted and justification imputed the moment we place faith in Christ who was delivered up to death for our offenses and raised for our justification (Rom 4:25).
As God amply illustrated through the Old Testament sacrificial system, without the shedding of blood there is no remission of sins (Heb 9:22; Matt 26:28). Forgiveness and justification are only possible through faith in His blood which propitiated our sins before God (Rom 3:25-26). That is why Jesus after His resurrection said, “he who does not believe will be condemned” (Mark 16:16). He also said that, unless one believes, they will die in their sins (Jn 8:24).
While it is true that Christ is the propitiation for the sins of the whole world and that in time Christ will have drawn all unto Himself, resulting in all being reunited in Him (Jn 12:32; Eph 1:10), I believe it is a grave omission to tell the unrepentant that they are already forgiven without the need of repentance and faith on their part. Instead of telling them that they are already forgiven, we should be saying to them, just as Paul said, speaking to the men of Athens, that God “now commands all men everywhere to repent, because He has appointed a day on which He will judge the world in righteousness by the Man whom He has ordained” (Acts 17:30-31).  Instead of telling them that they are already saved, we should say as he did: “Believe on the Lord Jesus Christ and you will be saved” (Acts 16:30-31).
 I further demonstrate that sin is against God in my blog: The Blurring of Biblical Distinctions.
 I examine the nature of Biblical repentance and the need for it in my blog True Repentance.