Why must there be a sacrifice for there to be forgiveness?

Because God is holy and righteous he must judge those who do wrong. That includes all of humanity. Because God is loving he provided a way of salvation through Himself to those who believe. Only the blood of the Godman could to take away our sins. Throughout the bible we see the signs. People in the world realize they are not perfect. Even if they don’t believe in God, they somehow know they fall short of something.

Romans 5:12

Sin entered the world through one man and death through sin, and in this way death came to all people, because all sinned. 

Romans 3:21

All have sinned and fall short of the glory of God. 

In the beginning Adam & Eve felt no shame in the garden. 

Genesis 2:21

After rebelling against God they felt shame and covered their shame with fig leaves. 

Genesis 3:7

Through an animal sacrifice, God provided a better covering for Adam and Eve. This was the first sign.

Genesis 3:21

God rejected Cain’s offering but accepted Abel’s offering because it was an animal sacrifice (another sign).

Genesis 4:1-4

God reveals life is in the blood and God required annual animal sacrifice for sins to be covered (another sign). This covering was a temporary system or a sign of a permanent covering that would come in the future

Leviticus 17:11 

Hebrews 10:1O

The prophet Isaiah spoke of this permanent covering that would come in the future. A man who would be pierced for our transgressions and crushed for our iniquities. In this prophecy we see the details of the crucifixion of Jesus that would take place over 700 years later (another sign). 

Isaiah 53:1-12

Jesus replaced the sacrificial system with an everlasting agreement. He is the Lamb of God who takes away the sins of the world. 

John 1:29

Therefore, you are not required to sacrifice lambs every year to cover your sins. If you confess with your mouth that Jesus is Lord and believe that God raised him from the dead you will be saved. 

Romans 10:9

Death comes through sin but the free gift of God is eternal life through Christ Jesus our Lord. 

Romans 3:23

John 3:16

For God so loved the world that He gave His only begotten Son that whoever believes in Him shall not perish but have eternal life. 

This belief in God is not just acknowledging His existence. Even the demons do this and they tremble in fear (James 2:19). You must put your complete trust in the blood of Christ for salvation which leads to a repentance that is not meritorious. Repentance viewed as a work for salvation is backwards thinking. Sometimes Protestant Pastors can sound like Roman Catholics regarding this issue. Satan doesn't care what denomination you belong to. His objective is to get you make you doubt and destroy your relationship with Christ. I know you have heard his voice speaking from the mouths of men. Repentance is the result of being saved; so therefore Repent. 

Ephesians 2:8

It is by grace we are saved through faith, not by good deeds that outweigh our bad deeds.