7 But He explains whence this should befall them: “For if you believe not that I am [He], you shall die in your sins.” I believe, brethren, that among the multitude who listened to the Lord, there were those also who should yet believe. But against all, as it were, had that most severe sentence gone forth, “You shall die in your sin;” and thereby even from those who should yet believe had hope been withdrawn: the others were roused to fury, they to fear; yea, to more than fear, they were brought now to despair.
But He revived their hope; for He added, “If you believe not that I am, you shall die in your sins.” Therefore if you do believe that I am, you shall not die in your sins. Hope was restored to the desponding, the sleeping were aroused, their hearts got a fresh awakening; and thereafter very many believed, as the Gospel itself attests in the sequel. For members of Christ were there, who had not yet become attached to the body of Christ; and among that people by whom He was crucified, by whom He was hanged on a tree, by whom when hanging He was mocked, by whom He was wounded with the spear, by whom gall and vinegar were given Him to drink, were the members of Christ, for whose sake He said, “Father, forgive them, for they know not what they do.”
And what will a convert not be forgiven, if the shedding of Christ's blood is forgiven? What murderer need despair, if he was restored to hope by whom even Christ was slain? After this many believed; they were presented with Christ's blood as a gift, that they might drink it for their salvation, rather than be held guilty of shedding it. Who can despair? And if the thief was saved on the cross—a murderer shortly before, a little afterwards accused, convicted, condemned, hanged, delivered—wonder not.
The place of his conviction was that of his condemnation; while that of his conversion was the place also of his deliverance. Among this people, then, to whom the Lord was speaking, were those who should yet die in their sin: there were those also who should yet believe in Him who spoke, and find deliverance from all their sin.
Source: Tractates on the Gospel of John (New Advent)