The OT High Priest would have an animal sacrifice for his sins first, then for the peoples sins.
For all have sinned & come short of Gods Glory. But God made a way for remission of our sins. Jesus said at the last Supper, "Drink this fruit of the vine, this is my Blood that is shed for the remission of sins of many. Eat this bread, this is my body that is broken for you.".
Without the shedding of blood (of a pure animal) there is no remission of sin. Jesus was the Lamb of God without blemish. He bore our sins on the cross. But since He Is without sin, death couldn't keep Him in the ground (the wages of sin is death). So He raised from the dead (so we also will raise from the dead a new everlasting body).
One man (Adam) we all inherit sin nature, sin & die. By one man (Jesus Christ) we can all be made righteous & have eternal life. It is only fair in the eyes of God.
Our sin separate us from God. God won't hear our prayers when we have sin. Sin is not believing God loves you. That may seem little. But it is doubting God. Without faith it is impossible to please God, for those who seek God, must Believe God Is, and is a rewarder to those who diligently seek Him.
Also, you probably have read 10 commandments and thought you never did any of these. But you have. When God shows you a sin, repent the sin to God (for those already redeemed) God is faithful & just to forgive you your sin and cleanse you from it.
Only those redeemed by His Blood have everlasting life in them. To see the Kingdom of God one must be born again, born of His Spirit, cleansed with His Blood, joint heirs with Jesus, part of the family of God.
Jesus Christ is 100% the Word of God. Jesus Christ died for us in His humanity, He is our High Priest in His humanity. His humanity is 100% human. The Word of God is of the three in one Yahweh Elohim; Father, Word & Holy Spirit. The Human is who could atone for our sins. He had to be like the 1st Adam before the fall, tempted like the first Adam, overcome Satans deception. Jesus in His Humity is without sin (He bore our sin on the cross). He overcame the devils deception as a human being (body, soul & spirit).
Jesus is the Lamb of God planned since the foundation, the Savior & Messiah planned since the beginning. He is the Elect and all who believe in Him & are redeemed by His blood are the Elect in Christ. All who believe in Him & receive Gods Redeeming Grace