Thanks for your question. It is a good one.
The answer is found in Hebrews 9:13-15.
"For if the blood of bulls and goats and the ashes of a
heifer, sprinkling the unclean, sanctifies for the purifying of the flesh, how much more
shall the blood of Christ, who through the eternal Spirit offered Himself without spot to
God, purge your conscience from dead works to serve the living God? And for this
reason He is the Mediator of the new covenant, by means of death, for the
redemption of the transgressions under the first covenant, that those who are called may
receive the promise of the eternal inheritance."
When Christ died on the cross, His blood not only
flowed forward to take care of the sins of those living under the new covenant but it
flowed backward to take care of the sins of those who lived and died under the old
covenant. Also, according to Romans 2:11-16
people will be judged by whatever law they lived under. Jesus said, the ones who
lived under Moses would be judged by the law of Moses.
I trust this will be helpful.
Leon Barnes