Title says it all. Also, just to start off, I am a believer but I always have questions like these pop up in my head when I read the OT. I was reading the Psalms tonight and the Sons of Korah and David both talk about their delight in the Lord for his forgiveness of sins and how He makes those who love Him righteous. If God forgave the sins of those who love Him before sending His Son, and made people righteous, then why did we need Jesus in the first place?
In the Old Testament people could obtain mercy and be forgiven for their sins, but they had to keep coming back again, and again to ask for mercy and forgiveness. That's why God instituted the blood sacrifice of bulls and goats and sheep, etc. The priest would offer up Sacrifices once a year for the sins of the people. And they had to keep doing this year after year because the blood of bulls can never completely cleans people of their sins. "For it is not possible that the blood of bulls and of goats should take away sins." (Hebrews 10:4)
But when Jesus came He totally obliterated sin so that if we believe in Him we are free from sin and totally reconciled to God forever. "By the which will we are sanctified through the offering of the body of Christ once for all.
And every high priest standeth daily ministering and offering oftentimes the same sacrifices which can never take away sins.
But this man, after He had offered one sacrifice for sins forever, sat down on the right hand of God.
From henceforth expecting till His enemies be made His footstool.
For by one offering He hath perfected forever them that art sanctified." (Hebrews 10:10-14)
Jesus was the last blood Sacrifice that needed to be made for the remission of our sins. We are saved forever and do not need to keep coming back every so often to continue asking for forgiveness and Salvation. We have it forever, "Once saved, always saved".
"And almost all things are by the law purged with blood, and without shedding of blood is no remission." (Hebrews 9:22)
In order for us to need to be forgiven and resaved again there would have to be another blood sacrifice for our sins. But Jesus was the last blood sacrifice because His Sacrifice completely cleanses us from all sins forever.
It is worth it to note that the old testament prophets also were saved by the grace of Jesus. They looked forward to Jesus' Sacrifice for them by Faith and we look back to What Jesus did for us. We're still all saved by Faith.