Excerpts From Translation of Rambam (Maimonides), Hilchot Issurei Biah Chapter 13
The proper way of performing the mitzvah is when a male or a female prospective convert comes, we inspect his motives for conversion. Perhaps he is coming for the sake of financial gain, in order to receive a position of authority, or he desires to enter our faith because of fear. For a man, we check whether he focused his attention on a Jewish woman. For a woman, we check whether she focused her attention on a Jewish youth.
If we find no ulterior motive, we inform them of the heaviness of the yoke of the Torah and the difficulty the common people have in observing it so that they will abandon [their desire]. If they accept [this introduction] and do not abandon their resolve and thus we see that they are motivated by love, we accept them, as [indicated by Ruth 1:18]: “And she saw that she was exerting herself to continue with her and she ceased speaking with her.”
For this reason, the court did not accept converts throughout the reign of David and Solomon. In David’s time, [they feared] that they sought to convert because of fear and in Solomon’s time, [they feared] that they were motivated by the sovereignty, prosperity, and eminence which Israel enjoyed. [They refrained from accepting such converts, because] a gentile who seeks to convert because of the vanities of this [material] world is not a righteous convert.
Nevertheless, there were many people who converted in the presence of ordinary people during the era of David and Solomon. The Supreme Sanhedrin would view them with skepticism. Since they immersed themselves, they would not reject them, but they would not draw them close until they saw what the outcome would be.
When a court did not check a [potential] converts background and did not inform him of the mitzvot and the punishment for [the failure to observe] the mitzvot and he circumcised himself and immersed in the presence of three ordinary people, he is a convert. Even if it is discovered that he converted for an ulterior motive, since he circumcised himself and converted, he has departed from the category of gentiles and we view him with skepticism until his righteousness is revealed.
Even if afterwards, [the convert] worships false deities, he is like an apostate Jew. [If he] consecrates [a woman,] the consecration is valid…
For this reason, our Sages said: “Converts are as difficult for the Jewish people to bear as a leprous blemish.” For most converts revert for some reason or other and cause Jews to stray. It is difficult to separate from them once they have converted. Look at what happened in the desert at the worship of the Golden Calf and Kivrot HaTa’avah. Similarly, most of [the complaints in the instances when] our people tried God were instigated by the mixed multitude.
Comment: During the time of Chief Rabbi of British Mandate Palestine, Rabbi Avraham Y. Kook, the pressure to accept converts with ulterior motives for conversion in Argentina was so intense, that there were rabbis from that country that decided not to accept converts at all. Rabbi Avraham Y. Kook approved of their decision, although he did not adopt such a practice for his own region.
Comment2: I will not hide the fact that the Rabbinical Council of America seems to have more lenient standards than the standard presented in this post on the issue of conversion for marriage. For their viewpoint see: http://judaismconversion.org/geirus-policies-and-standards/
Comment3: For those who wish to hear a balanced presentation explaining both the strict and lenient views on the issue of conversion for marriage, I refer my readers to Conversions for Marriage in America and Eretz Yisrael by Rabbi Mordechai Willig . In the audio he also expresses his view about the validity of the conversion of the most famous convert in America, today.