The requirements to minister in the outer court was that you must be of the tribe of Levi. Of course Aaron and his sons were Levites but God had made Aaron High Priest and choose his sons as the anointed and consecrated priests. This is the priesthood that has been replaced by the priesthood after the order of Melchizedek. First, we will see how one of the children of Israel, who was a covenant believer, could enter into the front part of the outer court, to the bronze altar, where the sacrifice was offered. God had said that the soul that sinned must surely die.
Sin always brings death.
That is a law of God. Therefore, the person that sinned immediately had the sentence of death upon him. However, God said that the one who had sinned was permitted to bring a substitionary sacrifice to die in their place. It had to be a clean and perfect sacrifice of the kind specified for sin. They could not bring money or fruit or anything except a blood sacrifice.