It was an instant type of confidence arising I think from past Kamma, in that before we had met the Buddha/Dhamma we had been practicing some Yoga techniques and we started to see the first two Noble Truths, but we didn't yet know the third and fourth. The yoga practice was insufficient to answer our deeper questioning because it didn't have enough methods and was mainly a concentration practice, which is inadequate to find the way out of Dukkha. We wanted a path really leading a way out of Dukkha rather than just calming the mind and reaching for an idea. Yoga could not give us the answers because we were not faith characters, we wanted to understand for ourselves. The first Buddhist book we picked up explained the Four Noble Truths and we became Buddhists right there and then, because this gave us the pathway leading out of Dukkha. And it was like a revelation, there is a path out of this Dukkha, there is a path out of ignorance.
The Four Noble Truths have been the basis for our practice until this day, and so when some people give us teachings, we want to apply them and see if they work to end our Dukkha. Some of the techniques that they gave us, we saw that they didn't work very well. However, when we picked up the Satipattana Sutta, we saw all these methods to train the mind, we started to apply them and found they worked. This caused a gradual experiential confidence to arise, which deepened our confidence and conviction.
Over the years, we have seen the Dhamma continue to work and when we started to see the transformation of our minds, that experiential confidence deepened further. In that way gradually leading to a very strong conviction.