Imagine telling your friends and loved ones that you are an atheist. Picture their reactions. Make it a worst case scenario. Not only will they be angry at you for becoming an atheist, but they'll be angry at you for lying as well. One other thing to keep in mind is that unless you only have a year or two left to live, you will be found out.
Nobody can keep up that kind of facade for very long.
In the long run, coming out will be significantly less straining than staying in. Coming out is a powerful way of promoting tolerance for atheists. Remember that your theistic friends and family have had their heads filled with stereotypes about atheists by their leaders and fellow theists. By coming out, you can show that these stereotypes are bull. Indeed, you may be the only atheist they know, and thus the only person capable of dispelling the stereotypes.