I'm not sure but the first song I heard must have been Wine of Aluqah or Birth of Venus Illegitima. Me and a friend went through his metal videos (long time ago now) on his PC at a LAN and we came across those two Therion videos. It was live from... Poland I believe? From 1998, but we saw these in 2002 or so. It wasn't the music that struck me first though, but the vocalist was just so fiiine (it was Martina Astner). The second thing that struck me was that the crowd sucked. The third thing that struck me however was that the songs were just plain awesome.
Nevertheless, I didn't get into Therion back then because I got the "wrong" albums. I should've bought Theli or Vovin but instead I got Lepaca Kliffoth. I like that album now but back then I was like "wow, won't go there". It turned me off on Therion for years, until a friend sent Uthark Runa to me and I recognized the band name and hey: "I kind of like this song". Some days after that I got Lemuria and Sirius B, then Theli and Vovin, and then the rest of the bands discography. And that's that.
PS. The song that however "sealed the deal" for me was Rise of Sodom and Gomorrah, such an intense song.