In Leviticus chapter four, verse three, God required a sin offering so those who sin in ignorance, out of weakness, or without deliberate intention could receive forgiveness. Intentional, rebellious sins, on the other hand, were to be punished by death in those Old Testament days. A trespass offering (similar to a sin offering) was for those guilty of a sin or injury which they can receive full restitution. Sin offerings were necessary for ritual cleansing as well. The sin offering foreshadows the atoning death of Christan and His bearing of punishment of our sins. His death, however, was infinitely more perfect than the Old Testament sin offering in that it provided a single atonement for all sin.
We, as New Testament believers, continually need the atoning blood of Christ to cover mistakes, weakness, and unintentional failings that flow from the frailty of human nature. The sins coming from a rebellious disposition against God and His Word will bring us under judgment and spiritual death unless we confess them and repent through a renewed faith in Christ’s atonement. The sin offering is called sin itself, and so is Christ, the antitype of it. Christ is most holy in himself who had no sin in him, nor knew any, nor were any committed by him. Yet, He appeared in the likeness of sinful flesh, took the place of sinners, and was their substitute, had all their sins laid upon him, and was by imputation made sin itself, and became an offering for it, and so fully answered the type of the sin offering.
Today, there is no need for sin offering because the Son of God bore our sins for all humanity. However, even though Christ redeem us from our iniquities, we still, as believers, have to ask God for forgiveness from any unknown sin that we might not be aware of daily.