Consider the context and the most probable sequence of events: The thief was on a cross, and he got there by sinning. He knew he wasn't coming down from that cross alive, because no one ever had. Concerned about his eternal destiny, it's intuitive he had pieced together what the Scriptures said of the Messiah (the Christ), because the debate of the week in Jerusalem centered on the person of Jesus. So the thief had already repented, and engaged the only person who could save him.
You're setting yourself up for a very rapid dismissal with this statement if you're attempting to sway nonbelievers.
Who else was on a cross? If being on a cross was de facto evidence of sinning, then what was Jesus Christ?
You've just lost that nonbeliever because the two plausible answers to that question are either that being on a cross is not evidence of sinning, or Jesus Christ was a sinner.
You'll appear to be illogical or even dishonest, unfairly yes, but that is the way nonbelievers will take that statement.