Examples from Scripture that refute the notion that Jesus' death on the cross reversed the Fall and changed man's sinful nature back to its original pre-fall state:

• Christ Himself warns His disciples against false prophets who will come and try to deceive even the elect (Mat 7:15; 24:11,24; Mar 13:22). This would have been unnecessary had men's fallen nature been rectified by Christ's death on the cross.
• The warnings of the apostles we read about AFTER Christ's death and resurrection should have been unnecessary too. But we read instead that people are as wicked as they have always been, and that "false teachers" and "false prophets" have come into the world. (Rom Chapters 1 and 2, 2 Pe 2:1; 1 Joh 4:1).

When it comes to the Scriptures, Mark Twain's remark applies in an interesting way:

"It is not what you do not know that will hurt you the most. It is what you think you know, but it just ain’t so!"