Abby's baby is the last baby to be born into the world - - the first baby to be born without a soul because the Hall of Souls (called "The Guf") is now empty - - so, a baby born without a soul cannot live - - this is the end of the world.
But, if there was never a soul available for this baby (until Abby died at the moment of his birth and gave her soul to the baby), then how did the baby live inside of her? There were ultrasounds showing the baby alive with heartbeat. Abby said that the baby was kicking constantly. So, the baby lived "in utero" without a soul, but at the moment of his birth, he died, because he never had a soul? The way that I understand it is that a soul is given to a baby at the moment of conception, not at the moment of birth.
The Guf was empty. Abby died at the moment of the baby's birth and gave her soul to the baby. Abby died and the baby lived. But Jesus then said that The Guf is full now. So, Abby died and the baby lived with Abby's soul - - but where did all of those other souls suddenly come from? Why couldn't the souls be shifted around somehow so that Abby could also live - - why would this baby be given a soul, to have to live without a mother?
It was a sacrifice for Abby to give her life for her baby and, therefore, save the world from ending. However, Abby was suicidal - - we see the scars on her wrists from a previous attempted suicide and we see her almost attempt suicide again in the movie. Even Jesus said that she lacked hope. It would have made more sense for a woman who valued her life to make that sacrifice.
I saw that one reviewer wrote that he never heard of The Guf and he believes that it does not exist. I copied this from Wikipedia regarding The Guf:
The Guf - - The Tree of Souls :
According to Jewish mythology, in the Garden of Eden, there is a Tree of Life or the "Tree of Souls" that blossoms and produces new souls, which fall into the Guf, also known as the "Treasury of Souls." Gabriel reached into the treasury and took out the first soul that came into his hand. Then Lailah, the Angel of Conception, watched over the embryo until it was born.
According to Rabbi Isaac Luria, the trees are resting places for souls. Sparrows can see the souls' descents, explaining their joyous chirping. The Tree of Souls produces all of the souls that have ever existed, or will ever exist. When the last soul descends, the world will come to an end. According to the Talmud, the Messiah will not come until the Guf is emptied of all of its souls. In keeping with other Jewish legends that envision souls as bird-like, the Guf is sometimes described as a columbarium, or birdhouse. The mystic significance of the Guf is that each person is important and has a unique role which only he or she, with his or her unique soul, can fulfill. Even a newborn baby brings the Messiah closer simply by being born. The Guf will empty as the last soul is given to the last baby born into the world and then the world will end and the Messiah will come.