I think that most atheists are strong atheists. There is a huge stigma surrounding atheism, so it's kinda like jumping into a cold swimming pool. You have to dip your toes in a engulf yourself slowly, well, you don't have to, but it certainly feels that way for most I would say.
Maybe I am projecting my own view a bit too much, but that's how it was for me. If you live in America especially, declaring yourself an atheist is something the vast majority of people will not see favorably. However, for me as I furthers my knowledge in science, philosophy and history I began to see it's just simply foolish to follow any organized religion. The next step was looking at the infinite regress of a creation theory of any kind.
I do consider myself a very staunch atheist, however, with this clim we can not deal in absoultes, because we simply can not make those claims with any degree of intellectual honesty. I am passionate about the truth, and science is the best method available for discovering and explaining truths. So while I will never make the claim there absolutely is no God, it is possible. I don't believe it's possible, it simply is. We can make some pretty strong claims against creation because we can figure that the probability of creation is very very low. We can not disprove it is possible, but we can come very close to ruling it out. In fact the probability is so very low, that it just becomes mostly implausible.
My point is, I think it's all about confidence in what you're claiming. I became a strong atheist by researching both sides and educating myself as much as possible. As the creation theory became less and less plausible, I became a "stronger" atheist. The problem is the massive social stigma in my opinion. We have made great strides in reducing that stigma, and I believe we will eventually eliminate it. The sooner the better in my opinion as I actually see religion as toxic to our society in so many different ways. We will all be better off when we can all get away from this archaic nonsense.
I guess for me it was when i started studying history and I realized the christians didnt always look like good guys. I was agnostic in like 5th grade and then a depressed but strong atheist by the time i got to high school
What made me become a "Strong Atheist"? - The constant hammering of being told to have respect for those who wouldn't even dream of granting the same to me. My skin has become thinner not thicker over the years and I now lash out at everyone who oversteps that line without exception, friend or foe.
I grew up in a Baptist household, but I was never convinced from the start. My father was a preacher & he use to simulate the act of catching the holy ghost which looked like he had a seizure moment which was bullshit. My mother raised me as a single parent & she was never was big on religion. She use to tell me to choose if I wanted to believe in a god or gods or not. My grandmother forced me as a child to be baptized & once she realized that I never went to church or paid my church dues I ask her to stop paying them for me to discontinue being associated with her church. What makes me a strong atheist in which I also trust that all atheist are strong atheist is the fact that I have been & will always be an empirical individual who uses deductive reasoning & the scientific method to explain any and everything that I had questions about. I personally feel religion should only be referenced as the history of explaining how primitive men & women thought processes were during the bronze age of man used to explain phenomenon that they could not explain until the tool of science freed us with the power of knowledge using empirical evidence. If it was up to me if humans did not use the practice of using the scientific method & common sense in their daily lives. There should be penalties & punishment for using any religion to live by before the facts of empirical evidence.
Religions make hating a virtue. The evil people are clearly defined, god hates them and so should you. Doing what I can to help to eliminate the needless hatred and the needless suffering engendered by religious myths is my gift to the future. Just like the people in the past payed it forward so I can have what I have today I will do the same for my descendents. We seem to have gained a large amount of ground in a short amount of time.