I don't believe in God and I'm not religious, but when I mention how I don't believe in God to my family, it's as if they are disapointed in me. It makes me so deppressed to think they are sad due to something I am. How can I stop this feeling?

Don't be sad ....The main point of parenting is to raise strong independent adults.  The main point of your "teen years"  is to find out who you are... And NOT spent it being some fake as' shadow  of who people expect you to be.  You have a right to make some of your own decisions and find your own beliefs. .....Parents are also disappointed when their children stop believing in Santa Claus ....but they get over it ... Cos they do....  Cos they love and respect you :)

Mind you , I'm not a parent :) but I was once a teen .... So my answer counts :) cos I said so

Let's clarify a few things that may help.

The word religious most commonly indicates a spirit of reverence toward God.

So if you don't believe in God, It makes a lot of sense that you are NOT religious.

Disappointment indicates that your parents are not happy with the fact that you do not presently believe in God.  If they choose to be unhappy about something, that is both their choice and their right.

But it is about them and not you---although it definitely is affecting you as you indicated.  I suspect there are many ways in which they are in fact quite pleased at what you are, but that  specific cause of disappointment for them is leaking into their general attitude toward you at the present.

And while belief or non-belief in God is perhaps more important within a family that the belief that a certain color of paint is the perfect color to paint a given room, it's still in the same category---you "believe and are convinced" that one color is in fact best while they believe another particular color is best.

And they are not sad about something that you "are," they are sad about the current conclusion you have reached about God and the fact that you are (I assume) disinclined to participate with the family in religious ceremonies.

I certainly understand how you feel and why you feel that way; but if you think about things a little differently, you may feel better about what's going on.

A perplexing family issue indeed. Believing .. Is a matter of choice .. I'm thinking your family (as disappointed as they may be) have to get used to the idea that you have come to an age where this type of decision is yours to make. Give them time to adjust. You are young yet .. And haven't emotionally or spiritually matured yet. Just because you don't believe now, does not mean you NEVER  will .. Nothing is carved in stone. You  may or may not ever believe in God .. But that does not define your spirituality necessarily . 

They are not disappointed in you, but in your choice and your right not to believe in God. There is a significant difference between the two. Please understand that.

While you have that right, just remember it is just as much their right to believe and practice their religion. In doing so they are most likely going to invite you to participate in church events. It goes a long way in family relations if you occasionally join in to support your family. It does not mean you have to join the church.

For example my husband and I were both raised Catholic. We do not practice the Catholic. The church doesn't even recognize my marriage because my husband is divorced, and did not annul the first marriage, plus we had  civil ceremony. My mother-in-law thinks we will come back to the church. We won't. We have been married for nearly 20 years, and neither of us were practicing Catholics before we met. We will go to church with her on Christmas Eve, but that is out of respect for her, not the church.

What I am trying to say is stick to your beliefs, but be respectful of your family's as well.

Generally I find it a very good practice to not mention my religious lweanings to people unlkess they insist on my artulication of same.  I certainly never volunteer the information.  ,