As you point out, as a Christian I don't believe as you do but I agree that doesn't give me the right to be disrespectful or rude. In fact, the opposite should be true, for God wants me to love you just as much as He does (even if you don't believe in Him).
The Bible tells us to "always be prepared to give an answer to everyone who asks you to give the reason for the hope that you have." It then adds, "But do this with gentleness and respect" (1 Peter 3:15). I have found over the years that very little is ever gained by antagonizing people and usually much is lost. My wife is fond of saying that more bees are caught by honey than by vinegar! Perhaps your letter will encourage us to stop and be more thoughtful toward others.
At the same time, this doesn't mean I don't have the right to challenge your beliefs or urge you to reconsider why you reject God. Many who claim to be atheists reject God simply because they want to run their own lives. Could this be true of you?
Instead, I urge you to look at Jesus Christ. The Bible not only says God exists; it says He loves us, and He even became a man in the person of Jesus. Don't close your mind to Him, for He alone can change your heart and give you hope for the future.