From man's perspective, Jesus of Nazareth was slain approximately 2000 years ago. However, in the mind of God, Jesus has been slain since the beginning of time. Revelation 13:8 refers to Jesus as "the Lamb slain from the foundation of the world." God is not bound to what we perceive to be the present time. Almighty God knows the end from the beginning; He knows what will happen before it actually does happen (Isa. 46:9,10; Rom. 4:17). Man is bound to the present age since he is not omniscient like God.
And therein is the key to answering our question. Since God is not bound to the present and since He knew that Christ would be sacrificed for the sins of the world at Calvary, He saw fit to make forgiveness possible through Jesus' blood even to those living before Jesus' blood was literally shed.
I believe this view adequately harmonizes Hebrews 10:4 with the verses we considered yesterday. In other words, when the Hebrews offered a sacrifice in which forgiveness was said to be possible, their forgiveness was not based upon the shed blood of the animal, per se. Rather, their forgiveness, like ours, was based upon the shed blood of Christ (Eph. 1:7; Heb. 9:15), even though that blood was only shed in the mind of God at that time.
Romans 3:23-26 is an exceedingly important passage to reflect upon that ties these thoughts together - "For all have sinned and fall short of the glory of God, being justified freely by His grace through the redemption that is in Christ Jesus, whom God set forth as a propitiation by His blood, through faith, to demonstrate His righteousness, because in His forbearance God had passed over the sins that were previously committed, to demonstrate at the present time His righteousness, that He might be just and the justifier of the one who has faith in Jesus."
Paul indicates that if God had forgiven sins under the Old Testament strictly on the basis of animal blood, then God would be considered unrighteous. One of the reasons for Christ's sacrifice was to show that God was just when, in His forbearance, He forgave sins previously.
Some have wondered whether God actually forgave these past sins completely until His Son had given His life on the cross because of Paul's use of the phrase "passed over" (Rom. 3:25). I believe this phrase is simply another way of saying that God had forgiven their sins previously. The terminology seems to be interchangeable. The Hebrew parallelism in Micah 7:18, in conjunction with the Old Testament passages we considered yesterday, supply ample evidence of this truth.
Before we conclude, we should note exactly what God required of those who lived prior to Calvary before He forgave their sins in His forbearance. He required faith and obedience! The animal sacrifices offered showed God that the individual had faith and was obedient. Thus, God could forgive them, and still be a just God, because He knows that Jesus' "future" sacrifice would take place. It should be understood that God requires faith and obedience from us also if we are to be forgiven by the blood of Christ. Neither faith, repentance, nor baptism would save us if Jesus had not shed His blood. Likewise, those living under the Old Testament could have never had their sins forgiven through their animal sacrifices if Christ had not been sacrificed later.
In a nutshell, the blood of Christ makes forgiveness possible for all mankind at any point in time, and actual forgiveness was and is given to any individual who shows his faith by obeying God's law during the time in which he lived! For the Hebrews, that included animal sacrifices, and for us today, it includes faith, repentance, and baptism!