I first heard, and heard about, Mad Sin at Stavanger Punk Festival in 2004 (+/- 1 year). I left their concert with mixed feelings.
At the time, I considered myself a christian. I never was a particularly conservative one (actually I was quite the opposite - open-minded and trying to be as non-biased as possible), but still, I found Mad Sin somewhat offensive to my beliefs and, well, blasphemous. Mostly the first, I suppose. Feeling that I could not condone this, I removed myself from my prime spot right in front of the scene.
Still, I couldn't leave the concert area. I was there with my little sister, and was tasked with watching over her (she was 16 at the time). So I moved over to the bar, and watched the band from there. And as a couple of songs had passed, I was caught up in an internal maelstrom. The band was up there insulting things I believed in (and I felt insulted), but at the same time, the music was damn catchy. The feeling that I couldn't like the music, because I didn't like the image and opinions of the guys standing up there was being seriously shaken by the sheer coolness of the music. It didn't help that they looked cool doing it as well.
So I left the concert with mixed feelings. As time passed, the feelings largely dissipated, and my memories of the event became a haze. And so did my belief. Dissipate, that is. Then, some day last winter, I came over some music by them again. God Save the Sin, I think it was. And, well, I was sold. The music caught me instantly, and without my former religious beliefs to make a conflict, I found myself able to enjoy it.
And so I did, immensly. It has actually become one of my favourite bands, and I've had at least one album of theirs on my mp3 player (a small 512MB USB player) constantly since last summer. Only Machinae Supremacy has had longer, more constant space on that player.
I should probably end this writeup with recommending an album, and that would be their latest, named Dead Moon Calling. While the older music, at least back to God Save the Sin, is just as catchy, this album feels more refined and well-executed from a sound perspective.
A final note: Mad Sins music isn't in general that offensive to christians (as far as I can remember, there's only one song that's actually overtly anti-christian, Holy Vacation). What I found offensive was their stage antics.