All sin -past, present and future sins were placed on Jesus! This had to include all future sins otherwise everyone that died after the cross would never be forgiven. We are all after the cross. Because God is just, he will never punish the same sin twice, he has already punished Jesus!
You are forgiven even before you sin, you have already been cleansed! Grace looked ahead from the cross at all the sins that could ever be done and dealt with them, all, once and for all!!
Isaiah 53:5 "But he was wounded for our transgressions; he was crushed for our inquities; upon him was the chastisement(or punishment) that brought us peace, and with his stripes we are healed."
Psalm 103: 2-3, 12 "Bless the Lord, O my soul and forget not all his benefits, who forgives all your iniquity, who heals all your diseases... as far as the east is from the west, so far does he remove our transgressions from us.."
Sin NEVER reigns over grace!!