“My part in the Atonement is not complete until you join it and give it away. (T-5.IV.6)”
Jesus says repeatedly in A Course in Miracles that he is in charge of the Atonement, that his resurrection was the event that started the Atonement principle (not the crucifixion). He does not express any understanding for the skeptics among us about how his resurrection might have been a true happening. He never downplays his resurrection also. He simply assumes that, somehow, in some way, the resurrection happened, and it started the whole movement of Christianity that transpired from then on.
How do we “give away” the Atonement? We share. We don’t share indiscriminately, because we are not called to proselytize. We are told, in one vivid point, that sometimes a smile may be all that another person is ready to accept. Salvation has come for that moment.
Thus we accept the Atonement for ourselves; that is “joining” it. Then we give it away in whatever manner the Holy Spirit prompts. And when we are on the beam, we may give it away without even being aware of a special prompting. We will just act in line with Jesus’s plan, his plan of Atonement or correction to the ways of the world.