Your questions here are at the heart of a new evolution of spiritual thought—an evolution that may be challenging to traditional Christianity, but ultimately should only whet the thought processes of spiritual truth-seekers, whether Christian or not.
Sin is redefined in The Urantia Book as "deliberate disloyalty to Deity." Please read this section titled: FORGIVENESS OF SIN.
And later, in the Jesus Papers, the Master offers the following discourse on EVIL, SIN, AND INIQUITY, which you will likely find enlightening, and (IMHO) largely consistent with the teachings of Christianity. Sin and forgiveness as taught in The Urantia Book remain, with certain adjustments, pretty much as understood for generations. And all of the adjustments in understanding should be good news indeed to those who have labored under the burden of presumed "original" sin. Atonement however, is a larger issue...
In this discourse cited above, Jesus touches upon the idea of atonement, but for a more complete exploration of atonement, please see Paper 89: Sin, Sacrifice, and Atonement, in its entirety. This paper explains the evolutionary origins of the idea of sacrifice, of sin, and of atonement. These are concepts that were developed in mankind as our ancestors grappled with the realities of life on planet Earth. They had their place in our spiritual progression, but as we continue in our understanding of God and as we incorporate the continuing revelations of God into our mass consciousness, these ancient concepts require a more thoughtful and dispassionate review.
One of the reasons that Jesus incarnated on earth was to reveal the heavenly Father. He said: "He who has seen me has seen the Father." That's a strong statement. So, when we view Jesus' life-the way he lived, the way he treated people, the things he said about the Father-we are hard-pressed to entertain the Old Testament view of God as judge, as vengeful punisher, as a Deity who is so upset with his erring children that he has to kill one child to satisfy his wrath over the fallen ones. The revelation of the Father in Jesus life, and the revelation of God in the first parts of The Urantia Book are entirely inconsistent with the idea of atonement.
Jesus' life was one of love, of service, of forgiveness. He never taught this idea of atonement, nor did he ever portray the heavenly Father in this way, or refer to himself as a sacrificial lamb. I am not a Bible expert, but it is my understanding that even in the New Testament, Jesus did not refer to himself this way.Atonement can be a large stumbling block for traditional Christians; as you observe, "Christianity would seem empty" without the concept of atonement. But this does not mean that Christianity cannot evolve into a higher and more effective path to finding the Father. It simply means that truth-seeking religionists of any church might want to think deeply about the concept of atonement, and whether it really fits with the revelation of the Father as portrayed in Jesus' life.
Questioning the atonement doctrine is a good exercise. But it must be done with a mind free of fear, a mind that is open to new possibilities, a mind that is ready to find the loving God revealed in Jesus' incarnation. In addition, practicing the religion that Jesus himself practiced—the religion of personal spiritual experience with God-within—will definitely help the believer to gain a personal understanding of the unconditional love of God. It will never engender fear or worry that God is a vengeful or angry Creator, but will establish a solid and unshakable foundation of loving communion with the personal God of Love. After all, I think you'd agree that the ultimate goal of a spiritual life IS a relationship with God. The established churches-Christian or otherwise-do not necessarily promote this personal seeking, free of church rules and laws.
We have prepared a topical study on the subject of atonement. It is a condensation of Urantia Book teachings on the subject. You can explore it HERE. Each of the passages has a link to further reading in The Urantia Book. Please see also Christianity's Problem for a comprehensive assessment of the ways modern Christianity can improve and assist in the spiritual renaissance of our planet.Reconciling these very basic ideas of sin, forgiveness and atonement is a task that each truth-seeker must undertake for him/herself. The fact that you are reading The Urantia Book says to me that you are a truth-seeker. So, I suggest that you follow the links I have provided for you; read the pages carefully and prayerfully. Ask God to illuminate your understanding and guide your thoughts about these matters.
Thanks again for writing with this very important question. In The Urantia Book you have found a treasure...a treasure of revealed information for the modern times in which we live, and for the evolution of mankind's spiritual progression. It does take some courage when one has had a lifetime of spiritual teachings that portray God in a certain way, to begin seeing him in a new way. But it can be done, has been done, and will continue to be done by truth-seekers everywhere. This is spiritual evolution, and we all take part in it to the ultimate upliftment of global mankind by the furtherance of the Kingdom of God within each of us.
Please feel free to write again at any time!