"I used to be an atheist like you"

"... then I took an arrow in the knee." (Sorry, I just had to.)

Seriously, though, it seems to be a rather common tactic among many creationists, apologists and street preachers when they engage in an one-on-one conversation with an atheist, that they claim that they were atheists as well, but then they studied what Christianity had to offer and came to the logical and rational conclusion that it makes the most sense.

From a non-insignificant amount of experience I can tell that in many cases this is a lie, or at the very least a deliberate twisting of the truth. The majority of secular people who become believers in Christianity as adults did not, unlike these people claim, do it because of studying Christianity and all other points of view and coming to the rational conclusion that Christianity must be true and everything else false. No, in the vast majority of cases people become believers not because of rational decisions but because of emotional reasons. It's only afterwards (sometimes even years afterwards) that they try to rationalize it.

In many cases when you actually start talking with them and what they (allegedly) used to believe before they converted, you will find out that they were not, in fact, acquainted at all with skepticism, logic, science and the scientific method. You will quite quickly get the impression that rather than having studied all sides thoroughly and impartially, this person was simply proselytized and given a biased and narrow point of view, and this person believed it. They will commonly present all the stereotypical misconceptions about atheism, skepticism and science (as well as the different branches of science dealing with biology and the history of the Universe and life.)

I'm not sure why exactly they try this "I use to be an atheist too" tactic. Perhaps in the naive hope that they will give the impression that they somehow relate to the other person and convince them that they know and understand their perspective (when in fact they don't, especially if the other person is an experienced and well-studied skeptic who knows all the tricks in the book that apologists use.)