What does Romans 5:16 mean by 'not as it was by one that sinned, so is the gift'?
The gift is not parallel to Adam’s offense. Christ did not commit a sin to cover Adam’s sin; instead, just as Adam plunged the human race into condemnation by his one act of sin, Christ lifted the human race by virtue of His death on the cross.
Les explains that Romans 5:16 contrasts Adam’s one act of sin with God’s gift. Reading verse 15 first, he notes that if through the offense of one many be dead, much more the grace of God and the gift by grace, which is by one man, Jesus Christ, hath abounded unto many—that is, all that will believe. Then in verse 16, 'not as it was by one that sinned, so is the gift' means the gift does not operate the same way Adam’s sin did. Christ did not sin a sin that would somehow cover Adam’s sin. Instead, as Adam plunged the human race into condemnation by his act of sin, so Christ has lifted the human race by virtue of His death on the cross. The following phrase, 'for the judgment was by one to condemnation,' shows that every offspring of Adam—everyone, black or white, Jew or Gentile, makes no difference—is still a child of Adam and consequently under condemnation. The contrast is therefore between condemnation through Adam and the gracious gift through Christ’s finished work.
And not as it was by one that sinned, so is the gift: for the judgment was by one to condemnation, but the free gift is of many offences unto justification.