His "death has taken place for the redemption of the transgressions" (Heb 9:15). Christ "through the eternal Spirit offered Himself without blemish to God" (:14). He offered "Himself as a guilt offering" (Isa 53:10). This was prophetic as when "the blood of goats and bulls" (Heb 9:13) was sprinkled on "those who have been defiled, [to] sanctify for the cleansing of the flesh" (:13). "One may almost say, all things are cleansed with blood" (:22). These were the "transgressions that were committed under the first covenant" (:15) which "was not inaugurated without blood" (:18). Moses explained it as "according to the Law" (:19) as "'the blood of the covenant, which the Lord has made with you'" (Ex 24:8). "How blessed is he whose transgression is forgiven, whose sin is covered! "How blessed is the man whom the Lord does not impute iniquity" (Ps 32:1-2). Then Jesus cited, "'This cup is the new covenant in My blood'" (1Co 11:25). "Where a covenant is, there must be the death of the one who made it" (Heb 9:16). When a person writes a will he must die before the executor can lawfully distribute to the beneficiaries. "A covenant is valid only when men are dead, for it is never in force while the one who made it lives" (:17).
God "made Him who knew no sin to be sin on our behalf, that we might become the righteousness of God in Him" (2Co 5:21). He "committed no sin" (1Pe 2:22) so how could he become sin? He was "made . . . to be sin" (2Co 5:21) because he was, in terms of a figure of speech, a sinless substitute as a sin offering "on our behalf" (:21). Mankind was already experiencing the penalty of death and was separated from God. In order to take on the burden of humanity's transgressions he had to bear the fate of sinners and experience those consequences. "Christ redeemed us from the curse of the Law, having become a curse for us" (Gal 3:13) and was therefore treated as though he was a sinner. Because he was sinless it was obvious he wasn't dying his own death per se but was a substitute for the sinner who was guilty and couldn't save himself. At the cross "when the sixth hour had come, darkness fell over the whole land" (Mk 15:33) which symbolizes God's turnng away from the Son whom he had made "to be sin" (2Co 15:21). This is reflected in Jesus' voicing, "'My God, My God, why hast Thou forsaken Me'?" (:34). The sin debt "was hostile to us" (Col 2:14) and he "nailed it to the cross" (:14). Because Christ had become sin you'd think it had also died. However, "He has taken it out of the way" (:14) which explains why sin is still in the world. What it means is that it made a way to be "raised because of our justification" (Ro 4:25) so "we might become the righteousness of God in Him" (2Co 5:21).