Among the many topics addressed by the Book of Esther is the question of that which constitutes an accceptable conversion. The Talmud (Yeb. 24b) already questioned the propriety (and the sincerity) of those non-Jews who converted 'because the fear of the Jews had fallen upon them' (Esther 8, 17). Nevertheless, it concludes that post factum their conversion was valid.
This conclusion, which is echoed by the Rambam (הל' איסורי ביאה יג, יד) and the Shulhan Arukh (יורה דעה סי' רסח סעיף ב), is (ultimately) the basis for the minimalist position that views the acceptance of the commandments (קבלת המצוות) by converts as negotiable (based upon the comment of the Maggid Mishneh on הל' איסורי ביאה יג, יז). Some of the opposition to the RCA concordat with the Israeli Chief Rabbinate is based upon the fact that this position, which is most closely identified with the late Rishon le-Tziyyon R. Ben-Zion Meir Hai Uziel זצ"ל in שו"ת משפטי עוזיאל חיו"ד סי' נ"ה-נ"ח, will no longer be validated by the RCA.
My research into the laws of conversion long ago led me to the conclusion that independently of halakhic considerations (such as the view of the חמדת שלמה, R. Haim Ozer Grodzenski and the Rav) R. Uziel's opinion cannot be justified historically.
Since I plan to publish my findings as a longer monograph I hesitated to post on the subject. However, at the urging of colleagues, I've decided to summarize my conclusions here.
1) There is a rule in law that that which goes without saying, usually does. Hazal did not address the requirement on the convert to accept an observant lifestyle, because that was self-evident. Moreover, the only Judaism that existed until the early 19th century was halakhic Judaism, so there was nothing to discuss.
2) What did concern Hazal were the factors that motivated the convert. That is why the Bet Din serves as a screening committee. If the convert doesn't pass muster, he's not accepted ab initio.
If he passed muster, then בדיעבד he's deemed a Jew. This is because he or she joined the Jewish community and observed the commandments.
Unacceptable motivations, though, are more than of initial concern. According to Maimonides (above), a person whose motivations are suspect is placed on long term probation until his sincerity is established. Evidently, the convert's motivations also were a considered issue post factum (Rav Soloveitchik זצ"ל was of the opinion that such a person was obligated to observe the commandments, but as yet not allowed to marry a Jew. This, though, leads away from the realm of pure history.)
3) The idea that conversion was possible without dedication to religious observance was a result of the emancipation, when Jews no longer had to live in Jewish communities. At that juncture, the absence of any specific written statement that observance is categorically necessary for conversion was interpreted as allowing for just that. As with most argumenta ex silentio, this one too was historically weak.
I am absolutely not arguing that historical considerations should play an active role in Psak Halakha. On the contrary, as the Rav pointed out so many times. Halakhah has its own dynamic and orbit. To peg the Law to external elements would be tantamount to undermining its integrity.
I wanted to provide an historical explanation for the absence of documentary evidence for the requirement that a convert's motives be appropriate and include commitment to submitting to the yoke of the mitzvot. Use of history in such an ancillary mode, judiciously undertaken, seems to me to be perfectly legitimate.
In that connection, I though it apt to post a passage from a fascinating (and deeply affecting) article by Rabbi Dr. David Berger that touches on this last point:
In the realm of concrete decision-making in specific instances, it is once again the case that the impact of academic scholarship does not always point in a liberal direction. In other words, the instincts and values usually held by academics are not necessarily upheld by the results of their scholarly inquiry, and if they are religiously committed they must sometimes struggle with conclusions that they wish they had not reached....Some academics do not hesitate to criticize
and even mock such rabbis for their insularity and their affirmation of propositions inconsistent with scholarly findings, but on occasions like this the very same people are capable of deriding other rabbis for their intolerant refusal to ignore modern scholarship. (D. Berger, 'Identity, Ideology and Faith: Some Personal Reflections on the Social, Cultural and Spiritual Value of the Academic Study of Judaism,' Study and Knowledge in Jewish Thought, ed. H. Kreisel, Beer Sheva: Ben-Gurion University Press 2006, 25-26).