How do we know Abel's sacrifice was accepted by God?

Abel's sacrifice was accepted by God as a witness of his righteousness, as stated in Hebrews 11:4. This acceptance was based not solely on the act of the sacrifice itself, but because it was offered by faith according to God's revealed will. The testimony of Abel's offering points to Christ's imputed righteousness, denoting that Abel understood the significance of his act; it was not about the sacrifice itself, but about the faith in the Savior to whom it pointed. God testifying to Abel's gifts and accepting him first as righteous before his offering signifies the precept of grace that underlies salvation.