Most of today’s atheists define atheism as sort of an “a-theism”, that is, lack of belief. This grants the atheist a burden of proof advantage; it’s a non-claim, and non-claims require no support.
I don’t like this definition: most vocal atheists aren’t merely non-committal or unopinionated about God. No, they really think God doesn’t exist!! They think God is improbable, ridiculous, like a unicorn, or the Flying Spaghetti Monster, or the Flying Teapot. They think that God needs to be opposed, eradicated. They aren’t a-theists, they are athe-ists.
The vocal, anti-theistic atheists should be more honest. They should embrace the traditional definition (athe-ism), instead of the more recent, watered-down, a-theism*; they should deny God’s existence. And this sort of atheism does have a burden of proof, for it is making a claim.
Also, if an atheist claims that belief in God is irrational, unjustified, harmful, etc, they have a burden of proof as well regarding the irrationality claim, for it is a claim.
So, if you are anti-God, anti-Christian, anti-theist, and you are honest, and you really think God is like the Flying Teapot or the Flying Spaghetti Monster, then you should own up to positive atheism, strong atheism, or whatever the vogue term is for denying God’s existence. And you will need to meet some sort of burden of proof.
*The earliest use of a-theism I am aware is Anthony Flew’s use in his 1972 paper, “The Presumption of Atheism.”