teaching—to instruct especially rulers and the elite on the meaning of Christianity, of salvation, and other key theological concepts.

Today using force to convert is unthinkable. Why did this idea, that was used for centuries, pass away?

A couple disclaimers: this is an immensely complex topic, and I'm not an expert on post-medieval missionary history. That being said, my hunch is that it’s largely due to the rise of Protestant evangelical movements, especially the Great Awakening. Here we see a new stress placed upon the individual soul and upon religious experience. Conversion becomes a voluntary, individual turning to God. Once you have that understanding, the idea of forcing someone to convert becomes absurd.

https://christianhistoryinstitute.org/magazine/article/interview-converting-by-the-sword