I'm sure I've told at least most of this story before, but I doubt it has all been in one place.
I was raised Catholic and had a lot of contact with German-American protestants and Hindus. So, I got a reasonable education on the concept of a national faith. Some critical parts of Christian dogma never really stuck for me, for example, original sin, damnation of nonbelievers, and the bizarre models of the trinity that I thought were better read as distinct individuals. Of particular concern to the protestants was my world-accepting conclusion that religion served a mainly worldly function and that – all other things being equal – it did not really matter which pantheon you venerated.
When I was nine years old, I read The Hobbit and was fascinated by Tolkien's runes, so I got some books from the library and learned about the historical runic scripts. Around this time I was visited in a dream by a man pierced by a spear, who told me to seek out the "true runes". This character seems obvious to me now, of course, but I didn't take any particular meaning from it at the time. The image has stuck with me for 25 years.
In high school I developed an interest in Middle English, and read all of Sir Gawain and the Green Knight and most of the Canterbury Tales in the original language. In college I had the opportunity to study historical linguistics and Old English, and read a lot of old Anglo-Saxon works as well as all of Beowulf in the original. One of my major points of interest was in trying to see through the Christian insertions to the Saxon heathen heritage of the underlying text. During this time I also found an interest in English linguistic purism, which I regret I have not opted to demonstrate in this post. It was my hope to learn Icelandic next and then start pushing through the Old Norse canon, but real life intervened and I went out and got a job which of course cut into my free time for reading.
Over the years I dabbled a bit in various kinds of divination and occult stuff, aimlessly, and without really finding anything all that interesting. I never really had any reason to give up my Christian faith, and just looked at that as the sort of experimentation that anyone with curiosity about the universe should pursue. But, years after college I finally picked up some of the reading I hadn't got around to, starting with the Poetic Edda in translation. When I got to the story of Odin's ordeal on Yggdrasil and his taking of the runes, it became terribly clear who that rune-obsessed man with the spear had been. It was as though I had finally looked up from a map I was following and turned around to see the decades of wandering that had finally led me to that point.
So, it's not really clear to me when you would say I "became" heathen. Perhaps "I became heathen during the 20-year period from age 9 to age 29". I can identify a particular week during which I would agree that I said "okay, now this is the path I follow". But I had also spent the previous two decades relentlessly walking in that direction, without really knowing why, other than that I found all this Germanic lore terribly interesting and absolutely urgent for me to read.