How should we treat potential converts?

(16) “Turn back, my daughters, go, as I am too old to be with a husband. If I were to say: I have hope, even were I to be with a husband tonight, and also were I to bear sons” (Ruth 1:12).
“Turn back, my daughters, go” – Rabbi Shmuel bar Naḥmani said in the name of Rabbi Yudan bar Rabbi Ḥanina: In three places it is written here: “Turn back” (Ruth 1:8), “Turn back” (Ruth 1:11), “Turn back” (Ruth 1:12), corresponding to the three times that one rejects a convert. If he insists beyond that, one accepts him. Rabbi Yitzḥak said: “A stranger shall not stay the night outside” (Job 31:32). A person shall always reject with the left and accept with the right. “As I am too old to be with a husband […were I to be with a husband tonight]” – Rabbi Yoḥanan said: The Torah teaches you proper conduct, that one engages in marital relations at night and not by day. That is what is written: “In the evening she would come and in the morning she would return” (Esther 2:14), and it is written: “Were I to be with a husband tonight.”

The Gemara analyzes the baraita. The Master said in the baraita: With regard to a potential convert who comes to a court in order to convert, the judges of the court say to him: What did you see that motivated you to come to convert? And they inform him of some of the lenient mitzvot and some of the stringent mitzvot. The Gemara asks: What is the reason to say this to him? It is so that if he is going to withdraw from the conversion process, let him withdraw already at this stage.

(14) One should not think that Samson who saved the Jewish people, and Solomon King of Israel, who is called "the friend of God," married gentile woman who did not convert. Instead, the matter can be explained as follows: 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."

(2) When a person comes to convert, say to him, "What did you see that motivated you to come to convert? If you know that Israel, in this time, that the Jewish people are oppressed, eroded, (The words are from "Why is your strong one overthrown?" -Jer. 46:15) insane, and suffering. If he says, I know this and I will still become a part of your people, receive him immediately and tell him the principles of the faith, that God is one and idol worship is forbidden, and tell him a lot about this, and teach him a few of the lighter mitzvot and a few of the more stringent mitzvot, and teach him about some of the punishments of the mitzvot, as it says, "Before you came to learn this, if you ate the forbidden fat, you would not be punished with excommunication, you transgressed Shabbat and you were not punished with stoning; but now, if you eat forbidden food, you will be punished with excommunication; and if you transgress Shabbat you will be punished with stoning." Don't say too much about this, and don't get too specific either. And when you teach him about the punishment of the mitzvot, also teach him about the rewards of the mitzvot and teach him that in the doing of certain mitzvot that will merit life in the world to come. There isn't a complete righteous person, rather a master of wisdom, that does the mitzvot and understands them. And say to him: "You should know that the world to come is hidden from the righteous, who are Israel. As for what you see that Israel is in distress in this world, because it is not granted them to receive the abundance of good things in this world like other peoples, lest their hearts should be indulged loftily and they should go astray and squander the reward of the world to come. But God didn't bring to them too much destruction, so that they would not observe. Rather, all of the idolators stand. And you explicate this idea so that he feels the dearness of it. If he receives this, circumcise him immediately and wait until he is fully healed, and afterwards, immerse him without a barrier. (Some say that you cut or shave his hair and you cut his fingernails and his toenails before immersion.) And three learned scholars stand at his back and teach him some of the light mitzvot and some of the more stringent mitzvot a second time, and he stands in the water. And if it is a woman, women sit her in the water up to her neck and the beit din is outside and they teach her some of the easy mitzvot and some of the more stringent mitzvot and she sits in the water and afterwards, she immerses before them and they turn their faces and leave in order that they do not see her when she is getting out of the water, and they bless the immersion after she comes out of the water. And once he has immersed, it is as if he is a Jew, but if he goes astray, he is like an apostate Jew - he is still sanctified, and his marriage is sanctified.

The convert is not informed of all of the details of the punishments, but rather they’re given a general idea. They’re told that before conversion if they ate chelev (prohibited animal suet) or violated shabbat they were not liable for death, but after they will be. He explains that they don’t need to know all the details of the prohibitions “because their intention may be for the sake of Heaven,” indicating that too many details might scare off even pious converts, and that is not the intention.

(15) 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.

Rabbi Ḥelbo said: Converts are as harmful to the Jewish people as a leprous scab [sappaḥat] on the skin, as it is written: “And the convert shall join himself with them, and they shall cleave [venispeḥu] to the house of Jacob” (Isaiah 14:1). This alludes to the fact that the cleaving of the convert to the Jewish people is like a scab.

The issue is that other Jewish people may learn from the habits and customs of the past lives the convert “holds on” to or the convert may not be knowledgeable enough to properly observe mitzvot and other Jews may accidentally transgress because they relied on this convert’s flawed mitzvah observance.

(18) For this reason, our Sages said: "Converts are as difficult for the Jewish people to bear as a leprous blemish." For most converts convert for an ulterior motive and [later] 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.

Avraham HaGeir (the Convert) was a polemicist against Christians; he was killed al Kiddush HaShem (sanctifying God’s name). He is quoted once in the commentary of Tosafot: He explained that sincere converts are often more knowledgeable and meticulous in their mitzvah observance than those who are born Jewish. They are excited about taking on new mitzvot and do so with gusto. This makes people who are born Jewish look bad since it reminds God that they don’t always do His will.