Your use of the phrase "religious delusions"

For the avoidance of doubt, I was putting that in the mouth of Hypothetical Modern-Day Karl Marx rather than expressing my own attitude to religion. (In case you care: I am an atheist; my wife is an active Christian; I firmly disagree with all the religions I know enough about to have an opinion but don't think words like "delusion" are generally helpful for describing them.)

As for the use of religion in the Sequences, I think what's going on is this:

• Eliezer thinks it's really obvious, when one thinks clearly, that the usual religions are wrong.
• He expects most of his readers to agree and have similar reasons.
• On the other hand, there are plenty of religious people about, some of whom are very smart.
• So he uses religion as an example of something that convinces lots of people despite being very wrong.
• Of course some of his readers will disagree, but he anticipates less disagreement on religion than on other topics where he sees widespread wrongness.
• He doesn't spend time arguing against religion because (1) he expects most people who remain religious despite exposure to hardcore rational thinking to be basically unpersuadable and (2) discussions of religion have a way of taking over (a bit like discussions of hot-button political issues) and he didn't want everyone engaged in religious flamewars rather than discussions of other things.

That all seems reasonable (whether or not correct) to me.