Is it forbidden for jews to proselytize or is it just a custom not to? Is there an halacha that forbids us to proselytize? Wasn’t it a common practice in the past, up to the rise of the Roman Empire?
Shalom, Thank you for your question. The Torah was given to the Jewish people as an inheritance, and as such is particularity suited to us. We do not believe that all the nations should (or even could) keep the laws of the Torah that are particularly suited to the Jewish soul. In fact, this is so strong an idea, that even teaching the Torah to the nations is limited. On the other hand, unlike other religions we certainly believe that mankind has a place in G-d's plan, and all have a task to perform in world. One does not have to be a Jew to do G-d's will, nor to attain reward in the world to come. With this in mind, there is no desire on our part to convert the nations to Judaism. We believe that they can serve G-d as they are – by keeping the 7 laws of Noah, and supporting the Jewish people in their task to make the world a more G-dly place. So, according to Jewish law one should not encourage non-Jews to convert. In fact, even if a non-Jew shows interest in converting, we must first try to discourage them (politely) – and only if they persist and show true persistent desire do we accept them as converts. There were times in history when there were greater number of converts – but I am not sure that this was due to any encouragement from the Rabbinic world. Today also, with no movement to proselytise, we see large numbers of converts. So, to summarise, the Jewish people do not encourage conversion of non-Jews, and certainly do not actively proselytise. If a non-Jew is interested in our religion, we can encourage them keeping the Noahide laws. If they continue to show interest in conversion, of course they can be directed to any of the fine programs and Rabbis who run courses and help those wishing to becomes Jewish. Blessings.