Why are you doing this?
Several reasons. First, because my deep Christian faith was not healthy in a lot of ways. It made me ashamed for doubting and asking questions, for thinking too much, or too little. For not volunteering enough; for volunteering and then being proud of having done something. Depending on your stripe of Christianity, you may or may not believe this, but we were taught that "thinking lustful thoughts is the same as adultery" and "thinking hateful thoughts about your little brother is the same as murder". So I was a 17 year old MurderRapeAdultress freaking out about my sinful nature at the church lock in. I still carry weird baggage from that time in my life; when I got married, I was worried my husband would think I hated him if I had a shower after sex. Because I'd always been taught that women feel filthy and ashamed after sex, and that's why you needed to shower, and if you had proper, marital Jesus Sex then you wouldn't feel dirty. No one ever informed me that the shower was due to the aerobic nature of the activity. It was a stupid idea, and I have no idea why it stuck, or why it bothered me as an adult, but it did. I use this example because it is stupid. Little, individual ideas religion spreads like this can seem benign or silly, but they hurt people by the bucketload every day, especially when we pass laws based on them.
Second reason? I owe it to the awesome atheists, who never gave up on me, and socratically decimated my self-righteous, terrible arguments. And then waited the years it took for the penny to drop. (And to you, kid in my freshman biology class, whose Bad Religion T-shirt I reported to the study hall Overseer for "offending my religion", I'm so, so sorry. Can I buy you a beer?)
When atheists become Christians, they gain eternal life.
We have no evidence of this. When people become Christians, Christians claim they gain eternal life. But there's really no way of knowing for sure. Death is sort of brutal like that.
What does atheism offer for Christians who becomes atheists?
A worldview where "I don't know, how can we find out?" is an okay answer to a hard question, and being demonstrably wrong about a thing is as easy to fix as saying "wow. that was some solid evidence. You are right and I am wrong. Thanks for teaching me that."