They deny the Christian doctrine of Substitution, which states that Jesus died for you, to pay your penalty. The WT says that Jesus died to pay back what Adam lost.
Since the WT teaches that you can self atone when you die, most members never realize the importance of Justification... which is also denied to members.
Justification means that there is no debt. Instead they teach, that through will-power, you can grow to perfection even though scripture rather forcefully states that by the works of the Law shall no flesh be justified!
As soon as someone puts faith in their church affiliation and works, what God offers is off the table.
It is cruel irony that when a person seeks to identify THE F&DS, instead of Jesus, they become very unfaithful and indiscreet... as well as a Watchtower slave.