For me, it was joy. I was raised in the religion, and I always figured I'd be dying along with 6 billion others because I hated the meetings, the preaching work and the rules, morality and threats of 'Jehovah'. I did not want it to be true.
For others here, finding out it was false was a hard time to go through.
As well as asking which feelings you had at first (joy/sorrow/or add your own) I have a follow up question for those who found it hard- What was it that made the impending deaths of 6 billion people okay for you? Did you really think everyone else was 'evil' and deserved death? Did you not think on it too much, left it in God's capable hands? Something else?
I know most of us say we're glad it is false now, so if it was hard facing reality at first, what was your mind making of all the things we now see as negative when you were a part of it?