I think it is ludicrous to say JWs are not Christians because they do not accept the Trinity.
I agree.
What he rejected was the idea of "substitution"-that Christ died instead of us, that by his death Christ paid the penalty for sin for Adam's offspring.
Barbour denied that Jesus received the punishment for your sins and mine on the cross. That idea of vicarious punishment is at the heart of the Pauline version of the work of Jesus. I think that is the closest thing to an idea that is common to all christian denominations. It it usually the first one to be denied by "cults" like the Watchtower. Rutherford made the ransom to be about an Adam=Jesus legal transaction and robbed the doctrine of it's power... (puts evangelical hat off again)