For in that he died, he died unto sin once: but in that he liveth, he liveth unto God. "Died unto sin" certainly does not mean here, as some have taken it, died by reason of sin, or to atone for sin, but has the sense, elsewhere obvious in this chapter, of ἀποθνήσκειν, followed by a dative, which was explained under Romans 6:2.
Christ was, indeed, never subject to sin, or himself infected with it, as we are; but he "bore the sins of many;" "the Lord laid on him the iniquity of us all." He submitted for us to the condition and penalty of human sin; but, when he died, he threw off its burden, and was done with it for ever (cf.
Hebrews 9:28, "Unto them that look for him shall he appear the second time without sin unto salvation"). The purpose of thus describing the permanent life to God of the risen Christ is, of course, to show that the new life of us who are accounted to have risen with Christ must in like manner be permanent and free from sin.
"Quo docere vult hanc vitae novitatem tota vila esse Christianis persequendam, Nam si Christi imaginem in se repraesentare debent, hanc perpetuo durare necesse est. Non quod uno momento emoriatur caro in nobis, sicuti nuper diximus: sed quia retrocedere in ea mortificanda non liceat.
Si enim in coenum nostrum revolvimur, Christum abnegamus; cujus nisi per vitae novitatem consortes esse non possumus, sicut ipse vitam incorruptibilem agit" (Calvin). The next verse expresses this clearly.