Before converting to Islam, Bosnians were Christians. And it was a special kind of Christianity — Bosnian Christianity. There was just one problem: nobody recognized it. Both Catholics and Orthodox Christians considered Bosnians heretics. Serbs called them "evil heretics." Meanwhile, Bosnians called themselves the "good people" or "good Bosnians."

You can often find stečaks throughout the country (medieval tombstones from Bosnian-Christian traditions). They're among the few remaining artifacts from that era. In Sarajevo (for example, here or here) you'll sometimes spot them lying inconspicuously by the roadside with no markers at all.

Since medieval Bosnia was apparently full of heretics, something had to be done about it. Catholics regularly showed up with crusades and enslaved the locals. A single successful crusade could result in thousands of people being taken into slavery. The slave trade continued for several centuries. Dubrovnik didn't pass a law banning the sale of people until 1416, and even then it took a long time to see real results.

Shortly before the Ottoman conquest, Bosnians were forcibly converted to Catholicism and their church was effectively destroyed. So when they later adopted Islam, it wasn't really that surprising.