Judaism is one of the oldest religions, but there aren't many Jews compared to Muslims and Christians.
What are main reasons behind that?
There are many reasons why Jews have a low population. Depending on your point of view, different reasons will be "more true" than the others.
It says in the Torah that the Jewish people, will be as numerous as the stars in the sky and the sands by the sea. However, it also says that we will be a minority amongst the nations. So while we may be uncountable, we will remain small compared to other nations of our age.
While Judaism is generally understood to be a religion, it is also a nation. Comparing it to the population of Christianity and Islam then becomes an odd comparison, since Christianity and Islam is made up of members from many many nations. Using that standard, the Jewish people have a population in comparable size to other old nations such as Greece, Zimbabwe, or Senegal.
Outside of Israel, Judaism has never been a religion that proselytizes. (Tries to convert people) In contrast, Christianity and Islam try to convert as many people as possible. In the early years of Islam this was done by waging wars against nations, and with Christianity this was done by missionaries all over the globe.
Over the many centuries of Jewish existence, the Jewish people have been the subject of attack and ridicule. After the destruction of the first temple, the Jewish population was killed off so that only 35% of the population remained. After the holocaust, fully 66% of European Jews were killed. The many pogroms and exiles, as well as various campaigns of forced conversions, has over the centuries greatly reduced the Jewish population. In comparison, Christian and Islamic conflicts have only ever killed a maximum of ~10% of their population.
TofeeqAhmad, I appreciate your changes to the question. The answer, however, remains that mass murder and an aversion to proselytizing have prevented much growth. At our very founding, according to our tradition and the text of the Torah, 70 people went down to Egypt as a family/tribe and developed into a small nation within a nation. We grew by leaps and bounds. And then, during the Exodus, only 1/5 came out unscathed. Then, at Mt. Sinai, due to the sin of the Golden Calf (mere months after leaving Egypt), thousands more died. Over the course of the journey through the wilderness, we lost some individuals to war and more were lost to plague.
In the land, there were periods of relative tranquility, during which times there was some growth, but there were also many, many periods of war and exile, during which times the population fell, sometimes dramatically.
Later periods had ups and downs, but once Christianity developed into a full-fledged religion with an empire behind it, there was very little the Jews could do to grow their numbers. Even sustaining their numbers became a challenge. From pogroms to inquisitions (the most famous was the Spanish Inquisition at the tail end of the 15th century, but there were many, and they occurred in many places, including places now friendly to Jews, like Great Britain) to outright genocide, there really hasn't been a time when the Jews have had unlimited opportunity for growth.
Add to that the fact that, for most of our history anyway, we have not really outwardly proselytized, either by force or by reason, so our growth has always depended on our ability to naturally increase our numbers by our birthrate exceeding our deathrate. Since genocide, slavery, mass murder, and torture significantly alter that ratio, any significant growth we've experienced has historically been offset by massive losses.
And I thank you for putting me into a very depressed frame of mind this morning... :(
The LORD did not set His love upon you, nor choose you, because ye were more in number than any people—for ye were the fewest of all peoples
And the LORD shall scatter you among the peoples, and ye shall be left few in number among the nations, whither the LORD shall lead you away.
In the hakdama to Be'er HaGolah, the Maharal explains that the most intense kedusha (holiness) is always expressed in the smallest, or most condensed, manifestation in this world. Thus, the smaller the area of the mikdash (temple), the greater its level of kedusha. So too the Jewish people, as the holy nation, have the smallest manifestation in this world.
the Zohar in sefer devarim (forget exact place) asks this (why does scripture state that the jewish nation will be so numerous and yet other nations are more numerous) The zohar answers that though the nations in total are more numerous than the jews but the jewish nation is the most numerous nation because it is the only one who is a nation. all the other nations are mixed with each other so they are not actually one nation.
The Jews never started out having a very large population. In fact, when you read the Torah you see that the entire nation of Yisroel coming out of Mitzrayim is numbered only 600,000 men! (Of course, the Yidden were just starting out.)
Another reason, and probably a better one, is that the Holocaust greatly affected the population of Jews. Plus, you might be talking of Orthodox Jewish population, but there are a great number of Jews who just aren't religious or who don't know they are Jews.
Like mentioned in Islam , conversion of a non muslim to Islam is allowed, while Jews don't do so. This makes a great Difference. Most Muslim living in different countries have not yet visited the birth place of Islam, the reason being all of them being those converted to Islam.
On the other hand Jews are an Indigenous breed and hence are few.
Mass murders , Massacres , Holocaust, Strategic expulsions, Forced Conversion being other factors for same.
Jewish people dont high birth rate as islam and Christian religions.
Jewish 1-2 children
Christian 1-6 birth rate
I'm Christian and I can tell you most jewish people prefer to work hard and follow God other than have many children they can't manage.
Islam religion dons't care if they can manage it or not giving birth is normal to islam they believe god will take care of them.
Also jewish is not wide spared including never using force.