"I used to be an atheist"

"I used to be an atheist" is a trope, often used by religious apologists usually so they can make lots of money selling books on how they allegedly "saw the light", in which the speaker asserts that they were previously an atheist before converting to the religion they currently follow. The argument is usually that since they were once like you, the arguments and/or experiences that convinced them will likely convince you as well, and that you should follow the same path that they took.

Flaws
In essence, it is the friend argument, but made in reference towards your (former) self rather than a current friend. The trope is fallacious because since the person making this argument now thinks differently than they once did, it is virtually impossible for them to reliably describe the way they once were, due to the bias that the person now has that their current view is correct. This often makes their opinion of their former atheist self at least somewhat straw-manned. Indeed, said opinion is often extremely strawmanned, to the point of making the apologist's claim to have once been an atheist appear dubious.

Whenever someone claims to have once been an atheist, it is usually pretty clear from the way they describe how they (allegedly) were that their "atheism" was at least somewhat less than well grounded, if not outright made up. Frequently, they seem to define "atheism" as actions like not attending church, using porn, lashing out at authority figures, or other things that are not really atheism, but which their religion teaches are wrong. This is sometimes accompanied by comments like "I did all that because I was angry at my pastor", "I refused to accept that I need God in my life", or for bonus points, "because I didn't want to follow God's rules". Other times, their commitment to the idea appears to have been founded on rather shaky ground, such as teenage rebellion (this is rarely the case), or abuse at the hands of a religious authority.

Oftentimes, they claim that their "conversion" had come about upon meeting someone who didn't trigger the same reaction from them ("I befriended a Christian who told me that Christians weren't all like that, that my abuser would be punished after death, and that I could be free of all this anger by embracing Jesus."). For obvious reasons, such a situation is very unlikely in most countries, where they would be more likely to encounter different types of people who are religious, since theists are currently the majority almost everywhere on the planet.

History
The trope is almost as old as Christianity itself, with Paul playing up his (real or exaggerated) hatred and persecution of Christians before he himself converted. In his case, though, he converted from Judaism, another religion.

The "I am the best at repenting because I used to be the biggest sinner" line of argument also crops up with Augustine, who gives us the sob story that he converted to Christianity when he — despite being illiterate at the time — was commanded by an angel "Tolle! Lege!" and pointed towards a Bible, which he — miracle of miracles — read, devoured, understood, and which made him convert on the spot.

In more recent times, Christian apologists like C.S. Lewis, Kirk Cameron, and Lee Strobel have used their conversion stories in their writings. It also appears to be very popular on Christian websites, who try to convince others that their experiences (like near-death experiences) prove the existence of (the locally popular) God. In addition, they also seem to be very popular on anti-atheist websites, like the Freedom From Atheism Foundation, a webshite website/Facebook page on which right-wing Christians whine about nonexistent injustices done to them by atheists and how all atheists are a bunch of little Joseph Stalin clones, in a fashion not unlike MGTOW and women.