If he became sin, taking on our sins, and the cross was accepted as justice by God vis-à-vis the resurrection, then it is clear the remission of sins happened at the cross.
However, and I acknowledge this is a fine theological point and I could be wrong, but we access that "hiddenness in Christ" when we become heirs into the family. It's remitted for us, but to have access, we must be born into the family. This is why He is "the Way." And the "way in" is the same way Abraham got in -- by grace, through faith (
Rom 4).