A good question to ask is why people who believe God is a myth do not treat Him like one? If you believe God is as real as Santa, the Tooth Fairy, the Easter Bunny etc., what possible reason could you have for being so passionate about that disbelief? The obvious answer is because they know they're wrong; God isn't as fake as Santa.
Further evidence comes from the fact that many of these people actually refuse to capitalise God's name. They will say things like "your god", or even "Bible god". Yet these same people often have a strong enough grasp on English to know that names are always capitalised, such as Santa. I even remember one individual who had a peculiar habit of capitalising every word. He Would Consistently Type Like This, And Yet, When It Came To The Word god...
But why would you want to dishonour a fictional character in such a way? Have you ever seen me write Allah with a lower case "a"? And sure, I'll say "Him", "His" and "He" when referring to God, whereas I won't do the same for Allah, but this is because capitalising God's pronouns is an actual grammatical abnormality. It is a way of bestowing honour upon God that is not bestowed to other people, real or imagined. Refusing to do this for Allah isn't belittling him, but refusing to give him a peculiar honor.
So, the question remains as to why unbelievers go out of their way to dishonor a God they claim they don't believe in. Aside from arguing against Him in a way they do not argue against fictional characters, when they do debate, they further reveal a subconscious belief in Him by actively trying to offend Him.
This can be forgiven, but it will be uncomfortable. Humility is never easy. It will require not only listening to that subconscious belief in God that keeps nibbling at you, but also acknowledging that you have sinned against Him, and are deserving of His wrath. But see, as humble as that may be, your own humility is insufficient. God must also humble Himself. And He did! 2,000 years ago, the Son of God was so humble that He became a simple baby boy, born to a maidservant and adopted by her carpenter husband, and He lived a life of poverty before dying in the most painful and humiliating way imaginable. Why? Because that's the punishment you deserve. He took your punishment on the cross. And so by humbling yourself enough to confess Him as Lord, and believe in your heart that God raised Him from the dead, you will be saved. Or you can go on with the audacity of believing you could last in His courtroom. Your call.