We are not automatically forgiven just because Jesus died on the cross.
Jesus redeemed us - paid the price for our release from the bondage of sin. But we can refuse his free gift.
From
catholic.com/thisrock/2001/0105fea5.asp…
***When a first-century person could not pay his debt, he was often thrown into prison until the debt was paid. To further humiliate him, the debtor was forbidden to pay the debt himself, even if he came into some money while in prison. A relative had to come and pay the amount owed. This paying of the bill was called redemption. Now, look, before Christ came, we were all in a sort of debtor’s prison because of the sin of Adam. We could not pay the debt ourselves. A relative had to come and pay it for us. This may be one reason why Christ became man, became our relative by blood, so to speak.
In any case, once the debt was redeemed, the relative was then given the right to walk back to the prison cell, open it, and escort the debtor out of the prison. This second process-the application of the ‘right-to-leave’ that the relative had purchased for the debtor and the debtor’s willingness to be so publicly humiliated in being led out of the prison-is what a first-century person would have understood by the term salvation.
"Catholics believe that the world was redeemed by Christ’s sacrifice on the cross, but the second part of the process, the salvation part, depends partially on us. You see, even when Christ walks into the prison and takes us by the hand to save us-Christians almost universally call this initial step “baptism”-we still have to walk with him out of the cell. And we can let go of his hand at any time before we get to the door, if we are sufficiently cantankerous or too ashamed to be recognized as his kin. We certainly can lose (technically, give up) our salvation, as any good first-century Jew or Gentile would have realized.
“Anyway, a person isn’t fully saved until he’s actually crossed the threshold of the cell-for us, by analogy, the prison of this world-into the daylight-heaven. If you told a first-century person that they could be sure of their salvation, they would have looked at you as if you were speaking a foreign language.”***
-Tim-