Catholic theologian Ludwig Ott states, "*The doctrine of the Immaculate Conception is not explicitly revealed in Scripture*" (Fundamentals of Catholic Dogma, p.200). He also admits that the first monograph on this subject was written in the twelfth century (p.201). Because of this, I would argue Roman Catholics are forced to begin with the immaculate conception, and then secondarily seek to refer back to Scripture for support or implicit proofs. So if there is any development here, it is backward development. It is taking a developed concept and seeking to read it back into Scripture. This is exactly what Pius XII said to do in the papal encyclical *Humani generis*:"It is also true that theologians must always have recourse to the sources of divine revelation; for it is their duty to indicate how what is taught by the living magisterium is found, either explicitly or implicitly, in Sacred Scripture and in divine 'tradition'. " David King rightfully stated of this,"This language of Pius XII amounts to a prescription, not for exegesis, but for eisegesis, i.e. reading into Scripture and/or tradition the doctrinal pronouncements of the Roman magisterium. Scripture, not to mention tradition, is regarded as an afterthought. It is the Roman magisterium that becomes the *norma normans non normata*, and as such, acts as the *norma normans* (the norm that norms) for both Scripture and tradition. Rome makes herself the final standard for the adjudication of all doctrine. To paraphrase Pius XII above, *the magisterium defines, then the theologians find*. Rome's position is *de facto* that of *sola Ecclesia*, a law unto herself" (Holy Scripture Vol. 1, p. 244). This backward development expressed by Pius XII can be mistaken as Biblical study. For instance, In seeking out implicit scriptural proofs, appeals to the phrase "full of grace" as a translation of Luke 1:28 are offered. This text is a favorite of the New Catholic E-pologists, but is not warranted, being rejected in many modern translations and scholarship. Even the translation used by the New Catholic Answer Bible (NAB) translates the phrase "Hail favored one!" This of course, does not stop the insert writers (Dave Armstrong and Paul Thigpen) from translating the phrase "full of grace"in one of their *inserts* and using it as a proof text. Nor does "full of grace" make any sense in the context. As Eric Svendsen pointed out, "*Contextually, the reason Mary is 'highly favored' is because she 'has been elected by God to conceive the Messiah', not because of some intrinsic and permanent quality of grace within Mary*" (Who Is My Mother?, p. 129).
Roman Catholic apologists have to be called out on the Marian dogma / Trinity parallel. By doing so, it’s an opportunity to direct them solely to the Biblical text, and also exposes faulty argumentation. Primarily, it exposes that what determines truth in Roman Catholicism is not the Bible. The guiding principle for Roman Catholic doctrinal development is none other than the Roman Catholic Church. That which sets the boundaries for Romanism is Rome. The Bible functions as it does in all sola Ecclesia groups, as a vehicle to promote what it wants God to say, rather than what God has said.
I would like for folks to start with the logical errors / false argumentation used before moving to the Biblical basis for the dogmas, but please feel free to post as you see fit.