Rejections and revisions of the great doctrines of the person of Christ, the penal substitutionary work of Christ, and the justification of sinners by grace alone through faith alone ought to be examined not only on their own terms but also in relation to the doctrine of sin.
It is possible to so minimise and pare down the doctrine of sin, in relation to its effects on human nature and its just condemnation by a holy God, that really there is no need, or desire, or reason to suppose that Christ is both God and Man, and that his work has an essential legal character.
Sin? No problem, God will forgive it. No incarnation needed.
Sin? No problem, God is love and accepts you as you are. No Mediator needed.
Sin? No problem, just repent of it and follow Jesus' teaching. No atonement needed.
Sin? No problem, just stop doing bad things and start doing what God wants. No regeneration needed.
Sin? No problem, just start over again and keep trying. No justification needed.
Sin? No problem, stop beating yourself up. No eternal consequences. No wrath no come. No hell to avoid. No Saviour needed.
Don't neglect the biblical doctrine of sin or you will go astray on the biblical doctrine of the Saviour and his work.
This is but a 21st Century echo of that wise, learned, godly Scottish theologian William Cunningham:
All false conceptions of the system of Christian doctrine assume, or are based upon, inadequate and erroneous views and impressions of the nature and effects of the fall,--of the sinfulness of the state into which man fell; producing, of course, equally inadequate and erroneous views and impressions of the difficulty of effecting their deliverance, and of the magnitude, value, and efficacy of the provision made for accomplishing it.Historical Theology Vol. 2, p.43