Under The Law, God required animal sacrifice ... a work:
https://www.gotquestions.org/animal-sacrifices.html
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Why did God require animal sacrifices in the Old Testament?
God required animal sacrifices to provide a temporary covering of sins and to foreshadow the perfect and complete sacrifice of Jesus Christ (Leviticus 4:35, 5:10). Animal sacrifice is an important theme found throughout Scripture because “without the shedding of blood there is no forgiveness” (Hebrews 9:22). When Adam and Eve sinned, animals were killed by God to provide clothing for them (Genesis 3:21). After the flood receded, Noah sacrificed animals to God (Genesis 8:20-21).
God commanded the nation of Israel to perform numerous sacrifices according to certain procedures prescribed by God. First, the animal had to be spotless. Second, the person offering the sacrifice had to identify with the animal. Third, the person offering the animal had to inflict death upon it. When done in faith, this sacrifice provided a temporary covering of sins. Another sacrifice called for on the Day of Atonement, described in Leviticus 16, demonstrates forgiveness and the removal of sin. The high priest was to take two male goats for a sin offering. One of the goats was sacrificed as a sin offering for the people of Israel (Leviticus 16:15), while the other goat was released into the wilderness (Leviticus 16:20-22). The sin offering provided forgiveness, while the other goat provided the removal of sin.
Why, then, do we no longer offer animal sacrifices today? Animal sacrifices have ended because Jesus Christ was the ultimate and perfect sacrifice. John the Baptist recognized this when he saw Jesus coming to be baptized and said, “Look, the lamb of God who takes away the sin of the world!” (John 1:29). You may be asking yourself, why animals? What did they do wrong? That is the point—since the animals did no wrong, they died in place of the one performing the sacrifice. Jesus Christ also did no wrong but willingly gave Himself to die for the sins of mankind (1 Timothy 2:6). Jesus Christ took our sin upon Himself and died in our place. As 2 Corinthians 5:21 says, “God made him [Jesus] who had no sin to be sin for us, so that in him we might become the righteousness of God.” Through faith in what Jesus Christ accomplished on the cross, we can receive forgiveness.
In summation, animal sacrifices were commanded by God so that the individual could experience forgiveness of sin. The animal served as a substitute—that is, the animal died in place of the sinner, but only temporarily, which is why the sacrifices needed to be offered over and over. Animal sacrifices have stopped with Jesus Christ. Jesus Christ was the ultimate sacrificial substitute once for all time (Hebrews 7:27) and is now the only mediator between God and humanity (1 Timothy 2:5). Animal sacrifices foreshadowed Christ’s sacrifice on our behalf. The only basis on which an animal sacrifice could provide forgiveness of sins is Christ who would sacrifice Himself for our sins, providing the forgiveness that animal sacrifices could only illustrate and foreshadow.
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In the Age of Grace, God requires belief, a work, imo--something you have to do: The Philippian jailer asked Paul and Silas, “Sirs, what must I do to be saved?” (Acts 16:30). Paul and Silas responded, “Believe in the Lord Jesus, and you will be saved” (Acts 16:31).
