"I used to be an atheist..."
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I used to be an atheist... is an argument used by several apologists such as C.S. Lewis, Kirk Cameron, and Lee Strobel who claim that they were once atheists, before converting to their religion. It is also popular among other Christians, particularly on Christian Websites, who try to convince others that their experiences (like near-death experiences) prove the existence of God. The argument is that since they were once like you, the arguments or experiences that convinced them will likely convince you as well, and you should follow the same path that they took.
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 and 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!"[note 1] and pointed towards a bible, which he - miracle of miracles - read, devoured understood and which made him convert on the spot. Generations of theologians have read through the ensuing drivel and the similar tall tale of Muhammad is one of the cornerstones of Islam (though Muhammad never really claimed to have been a "heathen" or "atheist" prior to his chit-chat with the archangel, he did claim to have been illiterate prior to writing the Q'uran).
In essence, it is the friend argument, but made in reference towards your (former) self rather than a current friend.