For a conversion to be considered giyur k’halachah, in accordance with Torah law, the potential convert must 1) accept that all mitzvos are Hashem’s commandments, and 2) agree to fulfill them all right away. Those “rabbis” who perform conversions knowing full well that these two basic requirements have not been met are machshil es harabim, causing many people to stumble, by having them believe that people who are in fact not Jewish, are Jewish. About those performing such conversions, the Gemara says, “Ra’ah achar ra’ah tavo lemekablei gerim”, their actions bring upon them disastrous repercussions. The halachah is that before a person is accepted for conversion their true motivation must be ascertained, and they have to be informed of at least of some of the mitzvos, including examples of mitzvos kalos and mitzvos chamuros, some of the minor mitzvos and some of the major mitzvos. However, if a convert had not been interrogated or informed of the mitzvos, the conversion may still be valid be’dieved, after the fact, and a rav should be consulted in order to evaluate the situation. But if the convert failed to be mekabel mitzvos, to accept all the mitzvos as commandments from Hashem, the conversion is totally invalid even after the fact. If there is even one mitzvah, including a Rabbinic mitzvah, that the convert is not willing to perform, or says they will take it on at a later time, the conversion is invalid.