The one thief was saved while he was on the cross.
Everyone who gets saved is saved in the same way, but there are as many ways of coming to salvation as there are people who have been born.
The one thief saw himself as deserving condemnation. He also desired to be pardoned. And, of course, he desired to be other than he was. He believed that God was full of loving kindness, and I think he may have even understood what the Lord Jesus was doing (at that very moment!) would justly satisfy this loving but absolutely righteous God. And finally he knew he needed to ask for this salvation, which he did.
Just like the rest of us who have received salvation.