It is worth noting that Pentecost means fifty, the fifty days after Easter. The old law was given fifty days after the Passover and the new law is given fifty days after Easter. We see this in the Acts of the Apostles,
When the day of Pentecost had come, they were all together in one place. And suddenly a sound came from heaven like the rush of a mighty wind, and it filled all the house where they were sitting. And there appeared to them tongues as of fire, distributed and resting on each one of them. (Acts 2:1-4)
This experience of receiving the Holy Spirit is obviously not the giving of a new written moral law (the Ten Commandants are always valid), but rather the old law is now written on human hearts. This is “inspiration” in the truest sense because inspiration literally means the going in of the Spirit. In that way the external law of Moses is fulfilled by the internal laws of Christ.
But he who looks into the perfect law, the law of liberty, and perseveres, being no hearer that forgets but a doer that acts, he shall be blessed in his doing. …. So speak and so act as those who are to be judged under the law of liberty. (James 1:25, 2:12)
For the law of the Spirit of life in Christ Jesus has set me free from the law of sin and death. For God has done what the law, weakened by the flesh, could not do: sending his own Son in the likeness of sinful flesh and for sin, he condemned sin in the flesh, in order that the just requirement of the law might be fulfilled in us, who walk not according to the flesh but according to the Spirit. (Romans 8:2-4)
To make an additional point, we know that in Old Testament times lambs (and other animals) were sacrificed in order to receive forgiveness of sins. However, the blood of the lambs could never change humanity inwardly.
“For since the law has but a shadow of the good things to come instead of the true form of these realities, it can never, by the same sacrifices which are continually offered year after year, make perfect those who draw near. Otherwise, would they not have ceased to be offered? If the worshipers had once been cleansed, they would no longer have any consciousness of sin. But in these sacrifices there is a reminder of sin year after year. For it is impossible that the blood of bulls and goats should take away sins. And every priest stands daily at his service, offering repeatedly the same sacrifices, which can never take away sins. But when Christ had offered for all time a single sacrifice for sins, he sat down at the right hand of God, then to wait until his enemies should be made a stool for his feet. For by a single offering he has perfected for all time those who are sanctified.” (Heb 10:1-4, 11-14)
But Jesus Christ is the true Lamb of God whose self-sacrifice on the cross and subsequent resurrection earned for humanity a genuine forgiveness of sins and reconcilement with God:
“Then he showed me the river of the water of life, bright as crystal, flowing from the throne of God and of the Lamb through the middle of the street of the city; also, on either side of the river, the tree of life with its twelve kinds of fruit, yielding its fruit each month; and the leaves of the tree were for the healing of the nations. There shall no more be anything accursed, but the throne of God and of the Lamb shall be in it, and his servants shall worship him; they shall see his face, and his name shall be on their foreheads. And night shall be no more; they need no light of lamp or sun, for the Lord God will be their light, and they shall reign for ever and ever. (Rev 22:1-5)
Or in another passage
“… they will make war on the Lamb, and the Lamb will conquer them, for he is Lord of lords and King of kings, and those with him are called and chosen and faithful.” (Rev 17:14)
So, when we look at all the history of the Bible we see Jesus Christ as the fulfilling of events and persons in the Old Testament. One of the most important fulfillments is that of the Paschal lamb. We know in the Old Testament as God was about to free the Hebrew people from slavery he order4ed the Jews to sacrifice a lamb and smear its blood on their door. In this way, the angel of God who was sent to slay the first-born sons of the Egyptians, would know not to kill the children of the Israelites inside the house. So, the death of the lamb in the Old Testament which saved the Hebrews from death foreshadowed the New Testament when the blood of Christ, the true lamb, saves all of humanity from death. St. John in his Gospel sys quite clearly that Jesus is the true lamb.
“… and he looked at Jesus as he walked, and said, “Behold, the Lamb of God!” (John 1:36)
It is interesting that in Western liturgies we find the phrases
“Lamb of God who takes away the sins of the world, have mercy on us. “Lamb of God who takes away the sins of the world, have mercy on us. “Lamb of God who takes away the sins of the world, grant us Thy peace.”
This takes place just before the breaking of the Eucharistic bread (i.e, the Body of Christ). The priest elevates it and says “The Body of Christ” so the people can see it and the priest continues “Behold the Lamb of God, behold Him who takes away the sins of the world. Blessed are those called to the supper of the Lamb.”
In these prayers we see clearly that we are saved from death by the sacrificial death of Christ, which foreshadowed in the Old Testament and is fulfilled in the New Testament and in the Liturgy of the Church.