Under the old covenant, they were never forgiven of sins because the high priest was a sinner and the sacrifice of animals could never take away sins.
The high priest died and went to the grave just like the people, therefore they were unforgiven.
Hebrews 10 “3 But in those sacrifices there is a yearly reminder of sins. 4 For it is impossible that the blood of bulls and goats should take away sins…..11 Every priest indeed stands day by day serving and offering often the same sacrifices which can never take away sins,”
But Jesus Christ never sinned and therefore He was the blameless lamb of God who died for our sins and took it away!
John 1 “29 The next day, he saw Jesus coming to him, and said, “Behold, the Lamb of God, who takes away the sin of the world!”
He rose from the dead because He is God Almighty! Therefore He took away our sins and our salvation is eternal because Jesus will never die again and He lives forever!
Hebrews 7 “22 By so much, Jesus has become the guarantor of a better covenant. 23 Many, indeed, have been made priests, because they are hindered from continuing by death. 24 But he, because he lives forever, has his priesthood unchangeable. 25 Therefore he is also able to save to the uttermost those who draw near to God through him, seeing that he lives forever to make intercession for them.”
He represents us in heaven as holy, blameless and perfect in the New Covenant that has forgiven us eternally!
Hebrews 10 “10 by which will we have been sanctified through the offering of the body of Jesus Christ once for all. ….17 “I will remember their sins and their iniquities no more.” 18 Now where forgiveness of these is, there is no more offering for sin.”
Jesus will not die again because your past, present and future sins are forgiven. He lives forever to give us eternal salvation that can never be revoked.