Isn’t God unjust in punishing us for Adam’s sin?
Not at all. Sanctifying grace was a free gift of God. No one has a right to a gift. When Adam sinned he forfeited his gift, thus losing it and God’s other gifts for us as well.
Now let’s say after your parents died you went to the bank and said to the banker, “Okay, here I am. Give me Mom and Dad’s money.” What do you think the banker will say? After he quits laughing, he’ll tell you all that money was lost by your parents, so it’s not there for you either. They lost their own wealth and, by necessity, lost what would have been yours. The same is true with original sin.