Presented almost as a diary, the book allows Mary to detail her sufferings in a first-person account. Eventually, through conversations with a widowed aunt named Winifred, she begins to discover the truth about heaven — that it is a very real and material place. "A new earth," her aunt says, quoting from the Bible. In fact, she uses various Bible passages and hymns to explain her belief not only to Mary but to others, including a Deacon, to prove that the afterlife is just a better version of life. These "spiritual lives" see sunrises, smell flowers, read books, talk with other spirits, build homes and families. As Winifred explains, her daughter Faith beside her:
A happy home is the happiest thing in the world. I do not see why it should not be in any world. I do not believe that all the little tendernesses of family ties are thrown by and lost with this life... Eternity cannot be—it cannot be the great blank ocean which most of us have somehow or other been brought up to feel that it is, which shall swallow up, in a pitiless, glorified way, all the little brooks of our delight. So I expect to have my beautiful home, and my husband, and Faith, as I had them here; with many differences and great ones, but mine just the same.
Phelps dedicated the book to her father Austin, a minister and educator, "whose life, like a perfume from beyond the Gates, penetrate every life which approaches it." The novel was extremely successful; one newspaper reported it earned the author $20,000 in one month (almost certainly an exaggeration). Whatever the numbers, the book was a huge seller, enthusiastically purchased by a country in grief after the recent horrors of Civil War. Several editions were printed, including an illustrated version with expensive Morocco binding. Phelps even produced a couple sequels.
Theologians were not as supportive of The Gates Ajar as the reading public. One critic in England wrote that the book's "wide circulation... does not speak well for the discrimination of its readers. It is simply a second-rate sensational novel, professedly of a religious character, but betraying so much positive error, and treating serious subjects in such a flippant, unhallowed strain, that no small amount of Christian charity is required to avoid the conclusion, that 'an enemy hath done this!'"