This is actually not an easy question to answer. The difficulty is that it is hard to come up with a good definition for "religion". Most definitions of religion do not go out far enough, meaning, there are beliefs that we would like to call religious but we cannot because the definition prevents us from classifying it as religious.
Let us begin with the simplest definition for religion. Religion is a belief in God or gods. This is the most common definition that people use. But it has a lot of problems with it. It is true that Judaism, Christianity, and Islam are believers in God, but that is not what makes them religions. Not all religions believe in God. For example, Buddhism and Scientology. I do not know much about Scientology but I do not think they believe in God, they believe in the evil lord Xenu who blow up all the aliens with an H-bomb inside a volcano something like 75 million years ago. I may be wrong about Scientology, maybe they do believe in God, but I doubt it, because they never mention God anywhere. Buddhism does not believe in God. There is no concept of God in Buddhism, it is a religion based on the reincarnation process of striving towards enlightenment. So immediately we have two religions that are not based on God or gods. This means that the simple definition above, that is, that religion is a belief based on God or gods is not a good definition. There is a second problem. There are people who believe in God, but they have nothing to do with religion. Back here I explained what deism is. For instance, Thomas Jefferson was a deist. He believed in God, but for him "God" was some abstract philosophical concept representing the First Cause, and that was essentially it. This is why a lot of atheists would support Jefferson's position towards religion because Jefferson himself was not religious, he was anti-religious. Thus, he is an example of someone who believes in God, but is not religious whatsoever. Again this illustrates that the definition "religion is a set of beliefs based on God or gods" is a bad one.
Deists do not believe in afterlife, while religions do, so perhaps we can define religion as "a set of beliefs concerning an afterlife". This is actually a stronger definition than the one above but it still has problems. Not all religions have an afterlife! Let us return back to Scientology. Again, I may be wrong, I do not know much about Scientology but I never hear them talk about afterlife. Scientology is about purging your body from thetans (dead alien material in your body) not about afterlife. But one can even use Judaism itself as an objection to that definition. Back here I wrote about my question concerning afterlife in Judaism. From what I understand originally Judaism did not have an afterlife, Rambam (Maimonides) himself writes that when people die they get resurrected in the future to live a second life, but life is NOT eternal. Thus, it seems that according to Rambam there is no eternal life upon death, while ancient Judaism seems to imply there is nothing after death. Therefore, defining religion as being concerned with an afterlife is not a good definition.
So what would be a good definition? I think I have found one. It is a complex definition. It is has many components to it. Religion is a set of beliefs so that: there is a end goal to be achieved, it promotes a way to live one's life (that is to say, it induces a moral code), it gives an emotional attachment to the believer, and if challenged by Reason or empiricism it shall not change its beliefs.
The curious aspect from the above definition, as we shall see soon, that religion can be something completely natural. And some supernatural belief can be non religious. For example, people who believe in the non-sense of astrology believe in some supernatural influence of the stars in the heavens over our lives. So let us check the list if astrology is a religion or not. It only satisfies only one requirement out of four. That is, if astrology is challenged by Reason or empiricism then the follower will ignore it because he likes to pretend astrology is true. That is the only component out of the four astrology satisfies. So even though it is a very irrational belief and supernatural based it nonetheless is not a religion. One can object to me and say that astrology promotes a way to live one's life. Strong believers in astrology follow what their made up horoscopes tell them. But even this component of the definition is not fully satisfied. This is because even though it promotes an astrologer a way to live a life it does not really create a moral code. An astrologer only follows his horoscope because he believes that will bring him good luck not because he has created a moral code to follow his horoscope. In some cases the emotional aspect in belief in astrology can also be satisfied. Regardless, astrology at best only satisfies two out of four requirements to be a religion, so it is not a religion.
Another example are people who believe in life after death but are not religious people themselves (that is, they do not follow any particular religion they just believe they will go somewhere after they die). This is NOT a religion. It certainly is not based on Reason or empiricism and if challenged few believers would actually reconsider their beliefs. It clearly has a strong emotional attachment to the believer, these people want to know they life on after they die, it gives them much comfort. However, it has no final end goal. These people are not striving towards anything. Nor do these people have any sort of moral code that promotes a way to live one's life. The same can be said about people who believe in ESP or people who believe in ghosts. These are silly beliefs but they are NOT religious.
Judaism is a religion. It is a set of beliefs that has a final goal, namely to follow the commandments of God so that he can send a Messiah to rescue the Jews from the Diaspora. It promotes a way to life one's life based on a moral code, which is the Torah and what the Rabbis say. Jewish believers have an incredible strong emotional attachment to their religion, sometimes tears will go down their face during the prayer of Yom Kippur. Finally, very few Jews will change their beliefs if someone carefully explains to them the problems with their beliefs based on Reason or empiricism.
Buddhism is a religion. It is a set of beliefs that has a final goal, namely to break the cycle of reincarnation by achieving Nirvana through enlightenment. It promotes a moral code to lives one's life, that is, the Four Noble Truths and the Eight Fold Path. It has a strong emotional hold on the believer of Buddhism. And finally it will not bow before Reason or empiricism.
Scientology is a religion. I will not go through the list again, it should be clear to the reader how to put it all together at this point.
Deism is not a religion (even though it believes in God). It is a set of beliefs, but it does not have any final goal. Deists do not try to attain anything they just live their life as if what they believe was non-existent. It does not promote any moral code. There are no prophets or holy books in deism. Deists and atheists together both follow a secular lifestyle which is usually based on just being a nice person to other people. It has no emotional attachment whatsoever, God does not answer prayers, God does not intervene with the world, there is no afterlife, and so on ... there is nothing emotional to be gained from deistic beliefs. And finally if challenged by Reason or empiricism deists would "convert" to atheists. Deists themselves say that something like the big bang theory points to that there has to have been a starting point, that is a First Cause, or they just use the First Cause argument to try to conclude that there has to be a starting point. Even if they are wrong about God they are at least rational about it.
What about atheism, is atheism a religion? There are some theists who say some atheists go too far and turn atheism into a religion. And it is true, some atheists do go very far. There are a lot of atheists who want the world to consist of atheists only and care about what religious beliefs a person has on a very important level to them. For example, there are plenty of atheists who said they want to have an atheist president. I think that is a stupid factor to have, having an atheist president does not mean anything about him, he can be a retard when it comes to economic issues. If having an atheist president is so important then one can elect themselves the next Joseph Stalin. What a person believes should not be the primary determining factor. What these atheists do is make it a primary determining factor. So, yes, they do go far with their atheism. But is it a religion? No. It does have a final goal, and that is for everyone to be an atheist. It does have a moral code, kinda, that is, those who are atheists are "good" and non-atheists are "bad". So they seek to "convert" everyone to atheism. But it has no emotional attachment and it cannot be called by Reason or empiricism because it is something based on Reason and empiricism. In general, atheism satisfies 0 out of 4 requirements, but the "atheist missionaries" only raise it to 2 out of 4, so it is not a religion.
I do want to add that just because "atheist missionaries" satisfy 2 out of 4 requirements it does not make it a 50% religion. No, that is not have this definition works. Something is either religion or not. It needs all four components to be satisfied. Having 3 out of 4 does not make it closer to a religion than 0 out of 4.