I dedicate two chapters discussing the Immaculate Conception of Mary in my book Behold the Mother of My Lord: Towards a Mormon Mariology, pp. 35-81, one chapter being dedicated to the purported biblical evidence (e.g., Luke 1:28) and one chapter on the overwhelming evidence against it from early Christianity.
I recently read a full-length work on the dogma. Here are some excerpts that are rather interesting:
The doctrine of the Immaculate Conception [unlike the Bodily Assumption], was raised as a question about the year 1100, and was not settled until 750 years later. (Edward O’Connor, “Preface” in The Dogma of the Immaculate Conception, ed. Edward D. O’Connor [Indiana: University of Notre Dame Press, 1958, 2016] vi; comment in square bracket added)
In order to become an official dogma of the Church a truth must have been revealed by God and handed down to the Church by the apostles, who are the foundation on which she is built. Yet many of the greatest theologians of the past were unable to discover the Immaculate Conception in the fonts of revelation, and some of them even considered it to be heretical. And not only the theologians, but even popes, when at various moments pressure was put upon them to decide the question, replied that “the Holy Spirit has not yet opened to His Church the secrets of this mystery” (Gregory XV, Epist. Ad monialem Regis catholici amitam, 4 June 1622) (Ibid., vi)
For anyone who thinks of the Church’s doctrine as a set of propositions of equal clarity, held by her in exactly the same way from the beginning, such a history poses an insuperable problem. Much of the difficulty vanishes, however, once we realize that the truths revealed by Christ are divine mysteries which the Church, another Mary, keeps in her heart and comes to understand even better and better over the centuries; and, on the other hand, that the history of the belief in the Immaculate Conception is not an erratic sequence of shifting fortunes in the struggle between conflicting views, brought finally to a halt by an arbitrary intervention of authority, but a steadily deepening realization of the holiness of the Mother of God, perceived dimly and imperfectly at first, but gradually asserting itself with increasing firmness and clarity. In this light, even the Parisian doctors of the thirteenth century are witnesses in favour of, rather than again, the Church’s belief; for they too concurred in attributing to Mary the greatest possible holiness under Christ. It was only because they did not see how an immaculate conception could be reconciled with the universality of the Redemption that they were hindered from pursuing their conviction about Mary’s holiness to its natural conclusion. (Ibid., vii)
[I]t is necessary that the objective inclusion of this truth in the primitive deposit to be manifested to us, not only by human authority (the exegetical, historical, and philosophical sciences) but by an authority divinely assisted in an absolutely infallible manner. In other words this truth must also be revealed subjectively (quoad nos). This is the “gnnoseological” element. (Charles Journet, “Scripture and the Immaculate Conception: A Problem in the Evolution of Dogma” in Ibid., 3-48, here, p. 4)
Such admissions (and the book is full of them) that the Immaculate Conception is a much later development and, ultimately, is believed due to the fact that Pius IX defined it as infallible dogma are rather revealing.
While writing about the Bodily Assumption, Karl Keating's comments are apropos for the Immaculate Conception, too:
Still, fundamentalists ask, where is the proof from Scripture? Strictly, there is none. It was the Catholic Church that was commissioned by Christ to teach all nations and to teach them infallibly. The mere fact that the Church teaches the doctrine of the Assumption as something definitely true is a guarantee that it is true. (Karl Keating, Catholicism and Fundamentalism: The Attack on “Romanism” by “Bible Christians” [San Francisco: Ignatius Press, 1988], 275; emphasis in original)
