If someone wants to start a car, he or she puts a key in and turns it (or these days, pushes a button). However, this is not the real reason the car starts. The reason the car starts is based on having a working engine with fuel in the car and a starter that turns the engine over. The key is merely the means of starting the car. The basis or grounds for starting the car are the engine and fuel.
In the same way, forgiveness of sins is not really accomplished by anything that people may do. Our actions in asking for forgiveness are merely means, not grounds. God is the one who forgives sins based on the death and resurrection of Christ Jesus.
Baptism is for the new believer the means for appealing for salvation (I Peter 3:21). Forgiveness of sins and the gift of the Holy Spirit are promised to those who repent and are baptized (Acts 2:38). But baptism is into the death of Christ Jesus (Romans 6:3), the real grounds of forgiveness. Ephesians 1:7 says of Christ, "In him we have redemption through his blood, the forgiveness of our trespasses, according to the riches of his grace,"
Similarly, the Israelites in the Old Testament were promised that God would forgive their sins when they sacrificed a sin offering (Leviticus 4:20, 26, 31, 35; 5:10, 13). But this was only a means. Hebrews 10:4 says, "For it is impossible for the blood of bulls and goats to take away sins." Rather, the sacrifices were shadows of the real sacrifice that actually takes away sins (Colossians 2:17; Hebrews 10:1). The grounds for the forgiveness of sins under the old covenant was the death of Jesus. Hebrews 9:15 says. "Therefore he is the mediator of a new covenant, so that those who are called may receive the promised eternal inheritance, since a death has occurred that redeems them from the transgressions committed under the first covenant."
This can be diagrammed this way:
|Means||Animal Sacrifices||Baptism into Christ|
Both animal sacrifices and baptism into Christ's death relate to his death, but it is that death of Christ that actually takes away sins.