How humans are "slaves", and how that leads to the existence of God.

[robvalue]

Irrational

It's the default option. Until you present clear evidence for the existence of God, atheism is the reasonable option to go with.
Keep in mind that atheism means lack of belief in God, not the belief that no God exists.

What made it a default puzzles me.

The default position for any claim should be a lack of belief. Otherwise, you believe everything you are told that can't immediately be proved wrong. Which is preferable?

The claim, "There is a god" should be treated with disbelief by default. As no evidence has yet emerged that the claim is true, continued disbelief is the logical position.

The claim "There is no god" should also be treated with disbelief by default. This puts you in the position of weak atheism when combined with the above.

Some may consider that the extraordinary nature of god, how it doesn't correlate at all with reality we observe, and how there is absolutely no evidence where you would expect to find it, to be enough to believe this second claim. Then you're in the position of strong atheism.

Some may say the concept is so ill-defined that statements about it are essentially meaningless (ignostic).

You're probably a strong atheist with respect to the Flying Spaghetti Monster. It's no different.