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This CHIASM is a contrast between those who practice hypocrisy, seen on the X element which is verse 6C, and the blood of Jesus His Son cleanses us from all sin, seen in the FIRST/LAST elements. The emphasis is:
1) The key to unlocking the FIRST/LAST elements, A and A′, is to see darkness as sin—darkness is a metaphor for sin. In Christ there is no sin (verse 5B) and His blood cleanses us from sin. In that way, we are conformed into the likeness of Christ, without the stain of sin.
2) The CENTER POINT reveals the problem with being a hypocrite, for our walk then does not match what we say.