Very interesting thread, thanks for reviving it.
In a way, I am one of those people who converted to Christianity from atheism.
I was born in 1957 in a family of two urban intellectuals (scientists) who lived in Kyiv (Kiev), Ukraine, back then part of the USSR. Officially, the old Soviet Union was a country where the freedom of religion was guaranteed by the 1936 Constitution, but unoficially it was a country where people with religious beliefs were never promoted, and often persecuted.
My parents were not "ideological," "convinced," aggressive Communists - in fact, my mom even managed to graduate from a university, get a job in science and work as a bench scientist (radiobiologist) without ever being a Communist Party member. My dad, largely because of his own father's strong Communist convictions, became a Communist Party member when he was in his early 20-s; however, as much as he was passionate about his work in science (electron microscopy), he was completely indifferent to anything in the sphere of politics or ideology. According to his own admission and also to testimonies of several friends, he always slept soundly during the Party meetings at his scientific institution.
My dad, as a Communist Party member, was formally required to be an atheist (not being an atheist was considered incompatible with remaining in the Party ranks). However, as far as I recall and as far as I knew him (and he was, in some respects, a very introvert person who would not easily get others in certain aspects of his life), he was an agnostic. He loved liturgical music - both the Orthodox liturgical chants or bell tunes, and the Western organ music, particularly by Bach. At a moment of delight caused by some beautiful piece of old Eastern or Western Christian music, he would sometimes close his eyes and say something like, "mmmm, heavenly, isn't it... there must be SOMETHING up there." On the other hand, he was always pretty sarcastic about churches, priests and "all that stuff."
My mom was an adamant agnostic and "universalist," or "ecumenist." When she was young, she became a fan of Tolstoy, and always retained the belief that "God's Kingdom is within us," which she interpreted as "whatever spiritual thing you believe in, is the same thing for all people; all faiths teach the same intrinsic, deeply humane, spiritual truths." Now, when she is 78, she is exactly the same in this regard as she was when I was little. She is curious, interested in all religions, but she would never agree with the notion that a person should choose ONE particular faith, because there is only one Truth. She, essentially, believes that all teachings about human kindness, goodness, self-sacrifice are equally truthful.
Older members of our extended family - my grandparennts' generation - were very diverse as far as faith went. My paternal (and only living) grandfather was a convinced Communist and a Communist functionnaire (educational bureaucrat), so he was pretty often vocally negative about religion, church etc. However, when he began to approach 80, he started to read a Russian Synodical translation of the Bible (later also a Ukrainian translation of the Bible, which he understood better), and expressed a sort of remorse about his militant atheist youth - especially that he, as a youngster, participated in demolishion of several old Orthodox churches. His wife, my paternal grandmother, said about herself that she was an "atheist," but she actually believed in all kinds of superstitions (evil eye, black cats, number 13, etc.). Her sister, my paternal grand-aunt (whom I always called simply "aunt Katya"), a widow of a military surgeon and a medical doctor herself, was a very devout Orthodox church goer. She lived in a different city, so I did not communicate with her on a regular basis, but I knew that she had the habit of going to church. She was an amazing woman - a true saint in everything; incredibly kind, soft-spoken, careful with people, always afraid of offending anyone, going extra mile to help with any chore, etc. However, when aunt Katya visited my grandparents or my parents and I was there at the kitchen or dining room table, she never spoke about God or faith - she knew that these things can harm me in my school, if my classmates or teachers found out.
Finally, my maternal grandmother, a woman whom I really loved for her outstanding intellect and ability to keep my attention for hours by reading and story-telling, was a very convinced militant atheist. She, a librarian by profession, loved everything French (spoke and wrote fluent French, had a ton of French books in her tiny one-room apartment), and her most favorite author was Voltaire. I have no idea why, but for some reason she had a very strong, focused, concentrated anger against religion and especially Christianity and Orthodoxy. She called Christians "g****mned idiots," and priests or monks "disgusting tricksters, deceivers and blood-sucking scumbags." If she saw a priest walking down a street, she would immediately curse aloud (even though she normally was extremely "cultured" and never swore), and spit on the ground several times. She used to say that the humankind made such a tremendous progress from barbarianism to its present day when we have cars, airplanes, telephones, and all this wonderful literature and arts; and it's just these crazy, stupid "religionists" who stand on the way of this wonderful progress.
My schools were, of course, rabidly anti-theist. From kindergarten to the senior high school year, and also throughout the university education I, like all other young Soviet people, constantly heard the message that there is no "god," that all these "gods" are stupid and evil fairy tales invented by the "exploiting classes" to keep the oppressed masses in fear, etc. etc. etc.
I became thinking about God, religion, Christ largely because I was very often bored and irritated by my schools. It was a rebellion thing in me. Still is.