So someone asked me the question yesterday, "What have you got to lose living life God's way?" I could have said I would have to give up alcohol, sex with my partner who is not married to me, or any of those other "worldly, sinful" desires, obviously. However, I responded with a sound financial reason to be an atheist.
This will deal with the scope of Christianity as the person who posed the question is a Fundie Christian, however, it can easily be applied to Islam as well, as it has a very similar (and actually, even stricter) requirement in this regard.
So, what have we got to lose? Well, recall that the Bible commands tithing, or giving up 10% of your gross income. Let's take a few examples to show just how much of a financial loss this could mean:
Example 1: You work a minimum wage ($7.25/hr) job, 40 hours per week, barely scraping by as it is. If you never advance and work this job for 30 years, that would be a total earnings of $452,400 over the course of 30 years. 10% of that is $45,240, which is exactly three years worth of wages gone. So you worked 30 years to make wages for 27 years worth of work, or put another way, you essentially worked three years for free. Ouch!
Example 2: You hold a $65,000/year salary job and work it for 30 years. Over 30 years, your earnings are $1,950,000. 10% of that is $195,000, which is exactly 3 years' salary. Same scenario as in the first example, but since you're pretty well off with that kind of salary, just think of what that money could have gone toward: retirement, savings, investments, or even something very nice for yourself.
Example 3: You run a thriving business and you have an obscene salary of $5,000,000/year, and your business lasts 30 years before you close it down. Over those 30 years, your earnings are $150,000,000. 10% of that is $15,000,000, which again, is exactly three years worth of salary. That $15,000,000 could have gone toward something much better than tithing.
In all three examples, we see we work for 30 years, but only get to keep what we make for 27 years worth of work. That is, every 10th year we work we get paid zero. Ouch!
So what are you losing "Living life God's way?" How about a full year's salary every 10th year of work? And that's just assuming a static salary, not taking into account raises and whatever. You take that into account, and you lose even more. In Islam, it's worse. The requirement is only 2%, but that's 2% of your TOTAL ASSETS. Depending on how much you have in real estate, personal items, etc., that can be a very, very sizable chunk of your paycheck.
Alright, so there's something I'd have to lose. All that money gone to waste. I'd like to see you Christians/Muslims/etc. try to refute that. There's the math, and it's absolutely irrefutable.